diff --git a/packages/backend/src/core/RemoteUserResolveService.ts b/packages/backend/src/core/RemoteUserResolveService.ts
index 5abc5934d2..38f45a03a4 100644
--- a/packages/backend/src/core/RemoteUserResolveService.ts
+++ b/packages/backend/src/core/RemoteUserResolveService.ts
@@ -74,7 +74,7 @@ export class RemoteUserResolveService {
 		if (user == null) {
 			const self = await this.resolveSelf(acctLower);
 
-			if (self.href.startsWith(this.config.url)) {
+			if (new URL(self.href).origin === this.config.url) {
 				const local = this.apDbResolverService.parseUri(self.href);
 				if (local.local && local.type === 'users') {
 					// the LR points to local
diff --git a/packages/backend/src/core/activitypub/ApRendererService.ts b/packages/backend/src/core/activitypub/ApRendererService.ts
index d7fb977a99..3a0367ba64 100644
--- a/packages/backend/src/core/activitypub/ApRendererService.ts
+++ b/packages/backend/src/core/activitypub/ApRendererService.ts
@@ -569,7 +569,7 @@ export class ApRendererService {
 
 	@bindThis
 	public renderUndo(object: string | IObject, user: { id: MiUser['id'] }): IUndo {
-		const id = typeof object !== 'string' && typeof object.id === 'string' && object.id.startsWith(this.config.url) ? `${object.id}/undo` : undefined;
+		const id = typeof object !== 'string' && typeof object.id === 'string' && new URL(object.id).origin === this.config.url ? `${object.id}/undo` : undefined;
 
 		return {
 			type: 'Undo',
diff --git a/packages/backend/src/core/activitypub/models/ApNoteService.ts b/packages/backend/src/core/activitypub/models/ApNoteService.ts
index 1891e10989..c61c15dccd 100644
--- a/packages/backend/src/core/activitypub/models/ApNoteService.ts
+++ b/packages/backend/src/core/activitypub/models/ApNoteService.ts
@@ -360,7 +360,7 @@ export class ApNoteService {
 			if (exist) return exist;
 			//#endregion
 
-			if (uri.startsWith(this.config.url)) {
+			if (new URL(uri).origin === this.config.url) {
 				throw new StatusError('cannot resolve local note', 400, 'cannot resolve local note');
 			}
 
diff --git a/packages/backend/src/core/activitypub/models/ApPersonService.ts b/packages/backend/src/core/activitypub/models/ApPersonService.ts
index 9dcdd874f8..56e4e521ed 100644
--- a/packages/backend/src/core/activitypub/models/ApPersonService.ts
+++ b/packages/backend/src/core/activitypub/models/ApPersonService.ts
@@ -225,7 +225,7 @@ export class ApPersonService implements OnModuleInit {
 		if (cached) return cached;
 
 		// URIがこのサーバーを指しているならデータベースからフェッチ
-		if (uri.startsWith(`${this.config.url}/`)) {
+		if (new URL(uri).origin === this.config.url) {
 			const id = uri.split('/').pop();
 			const u = await this.usersRepository.findOneBy({ id }) as MiLocalUser | null;
 			if (u) this.cacheService.uriPersonCache.set(uri, u);
@@ -285,7 +285,7 @@ export class ApPersonService implements OnModuleInit {
 	public async createPerson(uri: string, resolver?: Resolver): Promise<MiRemoteUser> {
 		if (typeof uri !== 'string') throw new Error('uri is not string');
 
-		if (uri.startsWith(this.config.url)) {
+		if (new URL(uri).origin === this.config.url) {
 			throw new StatusError('cannot resolve local user', 400, 'cannot resolve local user');
 		}
 
@@ -447,7 +447,7 @@ export class ApPersonService implements OnModuleInit {
 		if (typeof uri !== 'string') throw new Error('uri is not string');
 
 		// URIがこのサーバーを指しているならスキップ
-		if (uri.startsWith(`${this.config.url}/`)) return;
+		if (new URL(uri).origin === this.config.url) return;
 
 		//#region このサーバーに既に登録されているか
 		const exist = await this.fetchPerson(uri) as MiRemoteUser | null;
@@ -692,7 +692,7 @@ export class ApPersonService implements OnModuleInit {
 			await this.updatePerson(src.movedToUri, undefined, undefined, [...movePreventUris, src.uri]);
 			dst = await this.fetchPerson(src.movedToUri) ?? dst;
 		} else {
-			if (src.movedToUri.startsWith(`${this.config.url}/`)) {
+			if (new URL(src.movedToUri).origin === this.config.url) {
 				// ローカルユーザーっぽいのにfetchPersonで見つからないということはmovedToUriが間違っている
 				return 'failed: movedTo is local but not found';
 			}
diff --git a/packages/backend/src/core/activitypub/models/ApQuestionService.ts b/packages/backend/src/core/activitypub/models/ApQuestionService.ts
index d1936cfe1d..4e3df00e0d 100644
--- a/packages/backend/src/core/activitypub/models/ApQuestionService.ts
+++ b/packages/backend/src/core/activitypub/models/ApQuestionService.ts
@@ -71,7 +71,7 @@ export class ApQuestionService {
 		if (uri == null) throw new Error('uri is null');
 
 		// URIがこのサーバーを指しているならスキップ
-		if (uri.startsWith(this.config.url + '/')) throw new Error('uri points local');
+		if (new URL(uri).origin === this.config.url) throw new Error('uri points local');
 
 		//#region このサーバーに既に登録されているか
 		const note = await this.notesRepository.findOneBy({ uri });