diff --git a/packages/backend/src/core/NoteCreateService.ts b/packages/backend/src/core/NoteCreateService.ts
index 2a5fd2e1a6..b412d5db11 100644
--- a/packages/backend/src/core/NoteCreateService.ts
+++ b/packages/backend/src/core/NoteCreateService.ts
@@ -59,6 +59,7 @@ import { UtilityService } from '@/core/UtilityService.js';
 import { UserBlockingService } from '@/core/UserBlockingService.js';
 import { isReply } from '@/misc/is-reply.js';
 import { trackPromise } from '@/misc/promise-tracker.js';
+import { isNotNull } from '@/misc/is-not-null.js';
 import { IdentifiableError } from '@/misc/identifiable-error.js';
 
 type NotificationType = 'reply' | 'renote' | 'quote' | 'mention';
@@ -816,7 +817,7 @@ export class NoteCreateService implements OnApplicationShutdown {
 		const mentions = extractMentions(tokens);
 		let mentionedUsers = (await Promise.all(mentions.map(m =>
 			this.remoteUserResolveService.resolveUser(m.username, m.host ?? user.host).catch(() => null),
-		))).filter(x => x != null) as MiUser[];
+		))).filter(isNotNull);
 
 		// Drop duplicate users
 		mentionedUsers = mentionedUsers.filter((u, i, self) =>
diff --git a/packages/backend/src/core/activitypub/ApAudienceService.ts b/packages/backend/src/core/activitypub/ApAudienceService.ts
index d47be79441..0fccc7b950 100644
--- a/packages/backend/src/core/activitypub/ApAudienceService.ts
+++ b/packages/backend/src/core/activitypub/ApAudienceService.ts
@@ -8,6 +8,7 @@ import promiseLimit from 'promise-limit';
 import type { MiRemoteUser, MiUser } from '@/models/User.js';
 import { concat, unique } from '@/misc/prelude/array.js';
 import { bindThis } from '@/decorators.js';
+import { isNotNull } from '@/misc/is-not-null.js';
 import { getApIds } from './type.js';
 import { ApPersonService } from './models/ApPersonService.js';
 import type { ApObject } from './type.js';
@@ -40,7 +41,7 @@ export class ApAudienceService {
 		const limit = promiseLimit<MiUser | null>(2);
 		const mentionedUsers = (await Promise.all(
 			others.map(id => limit(() => this.apPersonService.resolvePerson(id, resolver).catch(() => null))),
-		)).filter((x): x is MiUser => x != null);
+		)).filter(isNotNull);
 
 		if (toGroups.public.length > 0) {
 			return {
diff --git a/packages/backend/src/core/activitypub/ApInboxService.ts b/packages/backend/src/core/activitypub/ApInboxService.ts
index 1cc54b6ff6..8d9cd74a25 100644
--- a/packages/backend/src/core/activitypub/ApInboxService.ts
+++ b/packages/backend/src/core/activitypub/ApInboxService.ts
@@ -27,6 +27,7 @@ import { QueueService } from '@/core/QueueService.js';
 import type { UsersRepository, NotesRepository, FollowingsRepository, AbuseUserReportsRepository, FollowRequestsRepository } from '@/models/_.js';
 import { bindThis } from '@/decorators.js';
 import type { MiRemoteUser } from '@/models/User.js';
+import { isNotNull } from '@/misc/is-not-null.js';
 import { getApHrefNullable, getApId, getApIds, getApType, isAccept, isActor, isAdd, isAnnounce, isBlock, isCollection, isCollectionOrOrderedCollection, isCreate, isDelete, isFlag, isFollow, isLike, isMove, isPost, isReject, isRemove, isTombstone, isUndo, isUpdate, validActor, validPost } from './type.js';
 import { ApNoteService } from './models/ApNoteService.js';
 import { ApLoggerService } from './ApLoggerService.js';
@@ -521,7 +522,7 @@ export class ApInboxService {
 		const userIds = uris
 			.filter(uri => uri.startsWith(this.config.url + '/users/'))
 			.map(uri => uri.split('/').at(-1))
-			.filter((userId): userId is string => userId !== undefined);
+			.filter(isNotNull);
 		const users = await this.usersRepository.findBy({
 			id: In(userIds),
 		});
diff --git a/packages/backend/src/core/activitypub/ApRendererService.ts b/packages/backend/src/core/activitypub/ApRendererService.ts
index 494622909a..d7fb977a99 100644
--- a/packages/backend/src/core/activitypub/ApRendererService.ts
+++ b/packages/backend/src/core/activitypub/ApRendererService.ts
@@ -315,7 +315,7 @@ export class ApRendererService {
 		const getPromisedFiles = async (ids: string[]): Promise<MiDriveFile[]> => {
 			if (ids.length === 0) return [];
 			const items = await this.driveFilesRepository.findBy({ id: In(ids) });
-			return ids.map(id => items.find(item => item.id === id)).filter((item): item is MiDriveFile => item != null);
+			return ids.map(id => items.find(item => item.id === id)).filter(isNotNull);
 		};
 
 		let inReplyTo;
diff --git a/packages/backend/src/core/activitypub/models/ApMentionService.ts b/packages/backend/src/core/activitypub/models/ApMentionService.ts
index 73eea1edf0..0ced7e88af 100644
--- a/packages/backend/src/core/activitypub/models/ApMentionService.ts
+++ b/packages/backend/src/core/activitypub/models/ApMentionService.ts
@@ -8,6 +8,7 @@ import promiseLimit from 'promise-limit';
 import type { MiUser } from '@/models/_.js';
 import { toArray, unique } from '@/misc/prelude/array.js';
 import { bindThis } from '@/decorators.js';
+import { isNotNull } from '@/misc/is-not-null.js';
 import { isMention } from '../type.js';
 import { Resolver } from '../ApResolverService.js';
 import { ApPersonService } from './ApPersonService.js';
@@ -27,7 +28,7 @@ export class ApMentionService {
 		const limit = promiseLimit<MiUser | null>(2);
 		const mentionedUsers = (await Promise.all(
 			hrefs.map(x => limit(() => this.apPersonService.resolvePerson(x, resolver).catch(() => null))),
-		)).filter((x): x is MiUser => x != null);
+		)).filter(isNotNull);
 
 		return mentionedUsers;
 	}
diff --git a/packages/backend/src/core/activitypub/models/ApNoteService.ts b/packages/backend/src/core/activitypub/models/ApNoteService.ts
index 8da9407216..e201b88173 100644
--- a/packages/backend/src/core/activitypub/models/ApNoteService.ts
+++ b/packages/backend/src/core/activitypub/models/ApNoteService.ts
@@ -37,6 +37,7 @@ import { ApQuestionService } from './ApQuestionService.js';
 import { ApImageService } from './ApImageService.js';
 import type { Resolver } from '../ApResolverService.js';
 import type { IObject, IPost } from '../type.js';
+import { isNotNull } from '@/misc/is-not-null.js';
 
 @Injectable()
 export class ApNoteService {
@@ -221,7 +222,7 @@ export class ApNoteService {
 				}
 			};
 
-			const uris = unique([note._misskey_quote, note.quoteUrl].filter((x): x is string => typeof x === 'string'));
+			const uris = unique([note._misskey_quote, note.quoteUrl].filter(isNotNull));
 			const results = await Promise.all(uris.map(tryResolveNote));
 
 			quote = results.filter((x): x is { status: 'ok', res: MiNote } => x.status === 'ok').map(x => x.res).at(0);
diff --git a/packages/backend/src/core/activitypub/models/ApPersonService.ts b/packages/backend/src/core/activitypub/models/ApPersonService.ts
index e80cd34a56..744b1ea683 100644
--- a/packages/backend/src/core/activitypub/models/ApPersonService.ts
+++ b/packages/backend/src/core/activitypub/models/ApPersonService.ts
@@ -38,6 +38,7 @@ import { MetaService } from '@/core/MetaService.js';
 import { DriveFileEntityService } from '@/core/entities/DriveFileEntityService.js';
 import type { AccountMoveService } from '@/core/AccountMoveService.js';
 import { checkHttps } from '@/misc/check-https.js';
+import { isNotNull } from '@/misc/is-not-null.js';
 import { getApId, getApType, getOneApHrefNullable, isActor, isCollection, isCollectionOrOrderedCollection, isPropertyValue } from '../type.js';
 import { extractApHashtags } from './tag.js';
 import type { OnModuleInit } from '@nestjs/common';
@@ -636,7 +637,7 @@ export class ApPersonService implements OnModuleInit {
 
 			// とりあえずidを別の時間で生成して順番を維持
 			let td = 0;
-			for (const note of featuredNotes.filter((note): note is MiNote => note != null)) {
+			for (const note of featuredNotes.filter(isNotNull)) {
 				td -= 1000;
 				transactionalEntityManager.insert(MiUserNotePining, {
 					id: this.idService.gen(Date.now() + td),
diff --git a/packages/backend/src/core/activitypub/models/ApQuestionService.ts b/packages/backend/src/core/activitypub/models/ApQuestionService.ts
index e78b3a3599..d1936cfe1d 100644
--- a/packages/backend/src/core/activitypub/models/ApQuestionService.ts
+++ b/packages/backend/src/core/activitypub/models/ApQuestionService.ts
@@ -10,6 +10,7 @@ import type { Config } from '@/config.js';
 import type { IPoll } from '@/models/Poll.js';
 import type Logger from '@/logger.js';
 import { bindThis } from '@/decorators.js';
+import { isNotNull } from '@/misc/is-not-null.js';
 import { isQuestion } from '../type.js';
 import { ApLoggerService } from '../ApLoggerService.js';
 import { ApResolverService } from '../ApResolverService.js';
@@ -51,7 +52,7 @@ export class ApQuestionService {
 
 		const choices = question[multiple ? 'anyOf' : 'oneOf']
 			?.map((x) => x.name)
-			.filter((x): x is string => typeof x === 'string')
+			.filter(isNotNull)
 			?? [];
 
 		const votes = question[multiple ? 'anyOf' : 'oneOf']?.map((x) => x.replies?.totalItems ?? x._misskey_votes ?? 0);
diff --git a/packages/backend/src/core/activitypub/models/tag.ts b/packages/backend/src/core/activitypub/models/tag.ts
index ced101b764..e7ceec3262 100644
--- a/packages/backend/src/core/activitypub/models/tag.ts
+++ b/packages/backend/src/core/activitypub/models/tag.ts
@@ -4,6 +4,7 @@
  */
 
 import { toArray } from '@/misc/prelude/array.js';
+import { isNotNull } from '@/misc/is-not-null.js';
 import { isHashtag } from '../type.js';
 import type { IObject, IApHashtag } from '../type.js';
 
@@ -15,7 +16,7 @@ export function extractApHashtags(tags: IObject | IObject[] | null | undefined):
 	return hashtags.map(tag => {
 		const m = tag.name.match(/^#(.+)/);
 		return m ? m[1] : null;
-	}).filter((x): x is string => x != null);
+	}).filter(isNotNull);
 }
 
 export function extractApHashtagObjects(tags: IObject | IObject[] | null | undefined): IApHashtag[] {
diff --git a/packages/backend/src/core/entities/DriveFileEntityService.ts b/packages/backend/src/core/entities/DriveFileEntityService.ts
index 50f1c49b48..8affe2b3bf 100644
--- a/packages/backend/src/core/entities/DriveFileEntityService.ts
+++ b/packages/backend/src/core/entities/DriveFileEntityService.ts
@@ -259,7 +259,7 @@ export class DriveFileEntityService {
 		options?: PackOptions,
 	): Promise<Packed<'DriveFile'>[]> {
 		const items = await Promise.all(files.map(f => this.packNullable(f, options)));
-		return items.filter((x): x is Packed<'DriveFile'> => x != null);
+		return items.filter(isNotNull);
 	}
 
 	@bindThis
diff --git a/packages/backend/src/core/entities/PageEntityService.ts b/packages/backend/src/core/entities/PageEntityService.ts
index fe7b137bd2..65c69a49a7 100644
--- a/packages/backend/src/core/entities/PageEntityService.ts
+++ b/packages/backend/src/core/entities/PageEntityService.ts
@@ -14,6 +14,7 @@ import type { MiPage } from '@/models/Page.js';
 import type { MiDriveFile } from '@/models/DriveFile.js';
 import { bindThis } from '@/decorators.js';
 import { IdService } from '@/core/IdService.js';
+import { isNotNull } from '@/misc/is-not-null.js';
 import { UserEntityService } from './UserEntityService.js';
 import { DriveFileEntityService } from './DriveFileEntityService.js';
 
@@ -102,7 +103,7 @@ export class PageEntityService {
 			script: page.script,
 			eyeCatchingImageId: page.eyeCatchingImageId,
 			eyeCatchingImage: page.eyeCatchingImageId ? await this.driveFileEntityService.pack(page.eyeCatchingImageId) : null,
-			attachedFiles: this.driveFileEntityService.packMany((await Promise.all(attachedFiles)).filter((x): x is MiDriveFile => x != null)),
+			attachedFiles: this.driveFileEntityService.packMany((await Promise.all(attachedFiles)).filter(isNotNull)),
 			likedCount: page.likedCount,
 			isLiked: meId ? await this.pageLikesRepository.exists({ where: { pageId: page.id, userId: meId } }) : undefined,
 		});
diff --git a/packages/backend/src/core/entities/UserEntityService.ts b/packages/backend/src/core/entities/UserEntityService.ts
index 53df32f210..14761357a5 100644
--- a/packages/backend/src/core/entities/UserEntityService.ts
+++ b/packages/backend/src/core/entities/UserEntityService.ts
@@ -25,6 +25,7 @@ import { IdService } from '@/core/IdService.js';
 import type { AnnouncementService } from '@/core/AnnouncementService.js';
 import type { CustomEmojiService } from '@/core/CustomEmojiService.js';
 import { AvatarDecorationService } from '@/core/AvatarDecorationService.js';
+import { isNotNull } from '@/misc/is-not-null.js';
 import type { OnModuleInit } from '@nestjs/common';
 import type { NoteEntityService } from './NoteEntityService.js';
 import type { DriveFileEntityService } from './DriveFileEntityService.js';
@@ -384,7 +385,7 @@ export class UserEntityService implements OnModuleInit {
 				movedTo: user.movedToUri ? this.apPersonService.resolvePerson(user.movedToUri).then(user => user.id).catch(() => null) : null,
 				alsoKnownAs: user.alsoKnownAs
 					? Promise.all(user.alsoKnownAs.map(uri => this.apPersonService.fetchPerson(uri).then(user => user?.id).catch(() => null)))
-						.then(xs => xs.length === 0 ? null : xs.filter(x => x != null) as string[])
+						.then(xs => xs.length === 0 ? null : xs.filter(isNotNull))
 					: null,
 				createdAt: this.idService.parse(user.id).date.toISOString(),
 				updatedAt: user.updatedAt ? user.updatedAt.toISOString() : null,
diff --git a/packages/backend/src/misc/is-not-null.ts b/packages/backend/src/misc/is-not-null.ts
index 584a09d35a..8d9dc8bb39 100644
--- a/packages/backend/src/misc/is-not-null.ts
+++ b/packages/backend/src/misc/is-not-null.ts
@@ -3,8 +3,6 @@
  * SPDX-License-Identifier: AGPL-3.0-only
  */
 
-// we are using {} as "any non-nullish value" as expected
-// eslint-disable-next-line @typescript-eslint/ban-types
-export function isNotNull<T extends {}>(input: T | undefined | null): input is T {
+export function isNotNull<T extends NonNullable<unknown>>(input: T | undefined | null): input is T {
 	return input != null;
 }
diff --git a/packages/backend/src/server/api/endpoints/gallery/posts/create.ts b/packages/backend/src/server/api/endpoints/gallery/posts/create.ts
index 784ae5088f..b07cdf1ed9 100644
--- a/packages/backend/src/server/api/endpoints/gallery/posts/create.ts
+++ b/packages/backend/src/server/api/endpoints/gallery/posts/create.ts
@@ -12,6 +12,7 @@ import type { MiDriveFile } from '@/models/DriveFile.js';
 import { IdService } from '@/core/IdService.js';
 import { GalleryPostEntityService } from '@/core/entities/GalleryPostEntityService.js';
 import { DI } from '@/di-symbols.js';
+import { isNotNull } from '@/misc/is-not-null.js';
 
 export const meta = {
 	tags: ['gallery'],
@@ -69,7 +70,7 @@ export default class extends Endpoint<typeof meta, typeof paramDef> { // eslint-
 					id: fileId,
 					userId: me.id,
 				}),
-			))).filter((file): file is MiDriveFile => file != null);
+			))).filter(isNotNull);
 
 			if (files.length === 0) {
 				throw new Error();
diff --git a/packages/backend/src/server/api/endpoints/gallery/posts/update.ts b/packages/backend/src/server/api/endpoints/gallery/posts/update.ts
index 8872b261dd..8bd83ff5ba 100644
--- a/packages/backend/src/server/api/endpoints/gallery/posts/update.ts
+++ b/packages/backend/src/server/api/endpoints/gallery/posts/update.ts
@@ -10,6 +10,7 @@ import type { DriveFilesRepository, GalleryPostsRepository } from '@/models/_.js
 import type { MiDriveFile } from '@/models/DriveFile.js';
 import { GalleryPostEntityService } from '@/core/entities/GalleryPostEntityService.js';
 import { DI } from '@/di-symbols.js';
+import { isNotNull } from '@/misc/is-not-null.js';
 
 export const meta = {
 	tags: ['gallery'],
@@ -67,7 +68,7 @@ export default class extends Endpoint<typeof meta, typeof paramDef> { // eslint-
 					id: fileId,
 					userId: me.id,
 				}),
-			))).filter((file): file is MiDriveFile => file != null);
+			))).filter(isNotNull);
 
 			if (files.length === 0) {
 				throw new Error();
diff --git a/packages/backend/src/server/api/endpoints/pinned-users.ts b/packages/backend/src/server/api/endpoints/pinned-users.ts
index 1f4509764f..784766bcb5 100644
--- a/packages/backend/src/server/api/endpoints/pinned-users.ts
+++ b/packages/backend/src/server/api/endpoints/pinned-users.ts
@@ -12,6 +12,7 @@ import { Endpoint } from '@/server/api/endpoint-base.js';
 import { MetaService } from '@/core/MetaService.js';
 import { UserEntityService } from '@/core/entities/UserEntityService.js';
 import { DI } from '@/di-symbols.js';
+import { isNotNull } from '@/misc/is-not-null.js';
 
 export const meta = {
 	tags: ['users'],
@@ -52,7 +53,7 @@ export default class extends Endpoint<typeof meta, typeof paramDef> { // eslint-
 				host: acct.host ?? IsNull(),
 			})));
 
-			return await this.userEntityService.packMany(users.filter(x => x !== null) as MiUser[], me, { schema: 'UserDetailed' });
+			return await this.userEntityService.packMany(users.filter(isNotNull), me, { schema: 'UserDetailed' });
 		});
 	}
 }