diff --git a/packages/backend/src/misc/convert-host.ts b/packages/backend/src/misc/convert-host.ts
index 856ce3c127..65cc3ca1f5 100644
--- a/packages/backend/src/misc/convert-host.ts
+++ b/packages/backend/src/misc/convert-host.ts
@@ -1,6 +1,10 @@
 import { URL } from "node:url";
 import config from "@/config/index.js";
 import { toASCII } from "punycode";
+import Logger from "@/services/logger.js";
+import { inspect } from "node:util";
+
+const logger = new Logger("convert-host");
 
 export function getFullApAccount(username: string, host: string | null) {
 	return host
@@ -13,6 +17,20 @@ export function isSelfHost(host: string) {
 	return toPuny(config.host) === toPuny(host);
 }
 
+export function isSameOrigin(src: unknown): boolean | null {
+	if (typeof src !== "string") {
+		logger.debug(`unknown origin: ${inspect(src)}`);
+		return null;
+	}
+	try {
+		const u = new URL(src);
+		return u.origin === config.url;
+	} catch (e) {
+		logger.debug(inspect(e));
+		return false;
+	}
+}
+
 export function extractDbHost(uri: string) {
 	const url = new URL(uri);
 	return toPuny(url.hostname);
diff --git a/packages/backend/src/remote/activitypub/models/note.ts b/packages/backend/src/remote/activitypub/models/note.ts
index 19b8fe49e7..3dc54e1fb5 100644
--- a/packages/backend/src/remote/activitypub/models/note.ts
+++ b/packages/backend/src/remote/activitypub/models/note.ts
@@ -1,6 +1,5 @@
 import promiseLimit from "promise-limit";
 import * as mfm from "mfm-js";
-import config from "@/config/index.js";
 import Resolver from "../resolver.js";
 import post from "@/services/note/create.js";
 import { extractMentionedUsers } from "@/services/note/create.js";
@@ -14,7 +13,7 @@ import { extractPollFromQuestion } from "./question.js";
 import vote from "@/services/note/polls/vote.js";
 import { apLogger } from "../logger.js";
 import { DriveFile } from "@/models/entities/drive-file.js";
-import { extractDbHost, toPuny } from "@/misc/convert-host.js";
+import { extractDbHost, isSameOrigin, toPuny } from "@/misc/convert-host.js";
 import {
 	Emojis,
 	Polls,
@@ -234,7 +233,7 @@ export async function createNote(
 				.catch(async (e) => {
 					// トークだったらinReplyToのエラーは無視
 					const uri = getApId(note.inReplyTo);
-					if (uri.startsWith(`${config.url}/`)) {
+					if (isSameOrigin(uri)) {
 						const id = uri.split("/").pop();
 						const talk = await MessagingMessages.findOneBy({ id });
 						if (talk) {
@@ -439,7 +438,7 @@ export async function resolveNote(
 		}
 		//#endregion
 
-		if (uri.startsWith(config.url)) {
+		if (isSameOrigin(uri)) {
 			throw new StatusError(
 				"cannot resolve local note",
 				400,
@@ -556,7 +555,7 @@ export async function updateNote(value: string | IObject, resolver?: Resolver) {
 	if (!uri) throw new Error("Missing note uri");
 
 	// Skip if URI points to this server
-	if (uri.startsWith(`${config.url}/`)) throw new Error("uri points local");
+	if (isSameOrigin(uri)) throw new Error("uri points local");
 
 	// A new resolver is created if not specified
 	if (resolver == null) resolver = new Resolver();
diff --git a/packages/backend/src/remote/activitypub/models/person.ts b/packages/backend/src/remote/activitypub/models/person.ts
index 1b7bd41bbf..6546f041b8 100644
--- a/packages/backend/src/remote/activitypub/models/person.ts
+++ b/packages/backend/src/remote/activitypub/models/person.ts
@@ -19,7 +19,7 @@ import { UserNotePining } from "@/models/entities/user-note-pining.js";
 import { genId } from "@/misc/gen-id.js";
 import { UserPublickey } from "@/models/entities/user-publickey.js";
 import { isDuplicateKeyValueError } from "@/misc/is-duplicate-key-value-error.js";
-import { toPuny } from "@/misc/convert-host.js";
+import { isSameOrigin, toPuny } from "@/misc/convert-host.js";
 import { UserProfile } from "@/models/entities/user-profile.js";
 import { toArray } from "@/prelude/array.js";
 import { fetchInstanceMetadata } from "@/services/fetch-instance-metadata.js";
@@ -138,7 +138,7 @@ export async function fetchPerson(
 	if (cached) return cached;
 
 	// Fetch from the database if the URI points to this server
-	if (uri.startsWith(`${config.url}/`)) {
+	if (isSameOrigin(uri)) {
 		const id = uri.split("/").pop();
 		const u = await Users.findOneBy({ id });
 		if (u) await uriPersonCache.set(uri, u);
@@ -166,7 +166,7 @@ export async function createPerson(
 ): Promise<User> {
 	if (typeof uri !== "string") throw new Error("uri is not string");
 
-	if (uri.startsWith(config.url)) {
+	if (isSameOrigin(uri)) {
 		throw new StatusError(
 			"cannot resolve local user",
 			400,
@@ -419,7 +419,7 @@ export async function updatePerson(
 	if (typeof uri !== "string") throw new Error("uri is not string");
 
 	// Skip if the URI points to this server
-	if (uri.startsWith(`${config.url}/`)) {
+	if (isSameOrigin(uri)) {
 		return;
 	}
 
diff --git a/packages/backend/src/remote/activitypub/models/question.ts b/packages/backend/src/remote/activitypub/models/question.ts
index 39963fddbf..59818654fd 100644
--- a/packages/backend/src/remote/activitypub/models/question.ts
+++ b/packages/backend/src/remote/activitypub/models/question.ts
@@ -1,10 +1,10 @@
-import config from "@/config/index.js";
 import Resolver from "../resolver.js";
 import type { IObject, IQuestion } from "../type.js";
 import { getApId, isQuestion } from "../type.js";
 import { apLogger } from "../logger.js";
 import { Notes, Polls } from "@/models/index.js";
 import type { IPoll } from "@/models/entities/poll.js";
+import { isSameOrigin } from "@/misc/convert-host.js";
 
 export async function extractPollFromQuestion(
 	source: string | IObject,
@@ -57,7 +57,7 @@ export async function updateQuestion(
 	const uri = typeof value === "string" ? value : getApId(value);
 
 	// Skip if URI points to this server
-	if (uri.startsWith(`${config.url}/`)) throw new Error("uri points local");
+	if (isSameOrigin(uri)) throw new Error("uri points local");
 
 	//#region Already registered with this server?
 	const note = await Notes.findOneBy({ uri });