diff --git a/packages/backend/src/core/MfmService.ts b/packages/backend/src/core/MfmService.ts
index 2200aeecea..42676d6f98 100644
--- a/packages/backend/src/core/MfmService.ts
+++ b/packages/backend/src/core/MfmService.ts
@@ -412,7 +412,7 @@ export class MfmService {
 			mention: (node) => {
 				const a = doc.createElement('a');
 				const { username, host, acct } = node.props;
-				const remoteUserInfo = mentionedRemoteUsers.find(remoteUser => remoteUser.username === username && remoteUser.host === host);
+				const remoteUserInfo = mentionedRemoteUsers.find(remoteUser => remoteUser.username.toLowerCase() === username.toLowerCase() && remoteUser.host?.toLowerCase() === host?.toLowerCase());
 				a.setAttribute('href', remoteUserInfo
 					? (remoteUserInfo.url ? remoteUserInfo.url : remoteUserInfo.uri)
 					: `${this.config.url}/${acct.endsWith(`@${this.config.url}`) ? acct.substring(0, acct.length - this.config.url.length - 1) : acct}`);