diff --git a/packages/backend/src/queue/processors/inbox.ts b/packages/backend/src/queue/processors/inbox.ts
index 51652357f..033f3413b 100644
--- a/packages/backend/src/queue/processors/inbox.ts
+++ b/packages/backend/src/queue/processors/inbox.ts
@@ -95,11 +95,25 @@ export default async (job: Bull.Job<InboxJobData>): Promise<string> => {
 	}
 
 	// HTTP-Signatureの検証
-	const httpSignatureValidated = httpSignature.verifySignature(
+	let httpSignatureValidated = httpSignature.verifySignature(
 		signature,
 		authUser.key.keyPem,
 	);
 
+	// If signature validation failed, try refetching the actor
+	if (!httpSignatureValidated) {
+		authUser.key = await dbResolver.refetchPublicKeyForApId(authUser.user);
+
+		if (authUser.key == null) {
+			return "skip: failed to re-resolve user publicKey";
+		}
+
+		httpSignatureValidated = httpSignature.verifySignature(
+			signature,
+			authUser.key.keyPem,
+		);
+	}
+
 	// また、signatureのsignerは、activity.actorと一致する必要がある
 	if (!httpSignatureValidated || authUser.user.uri !== activity.actor) {
 		// 一致しなくても、でもLD-Signatureがありそうならそっちも見る
diff --git a/packages/backend/src/remote/activitypub/check-fetch.ts b/packages/backend/src/remote/activitypub/check-fetch.ts
index e7d011d90..b583a4d7e 100644
--- a/packages/backend/src/remote/activitypub/check-fetch.ts
+++ b/packages/backend/src/remote/activitypub/check-fetch.ts
@@ -87,11 +87,25 @@ export async function checkFetch(req: IncomingMessage): Promise<number> {
 		}
 
 		// HTTP-Signatureの検証
-		const httpSignatureValidated = httpSignature.verifySignature(
+		let httpSignatureValidated = httpSignature.verifySignature(
 			signature,
 			authUser.key.keyPem,
 		);
 
+		// If signature validation failed, try refetching the actor
+		if (!httpSignatureValidated) {
+			authUser.key = await dbResolver.refetchPublicKeyForApId(authUser.user);
+
+			if (authUser.key == null) {
+				return 403;
+			}
+
+			httpSignatureValidated = httpSignature.verifySignature(
+				signature,
+				authUser.key.keyPem,
+			);
+		}
+
 		if (!httpSignatureValidated) {
 			return 403;
 		}
diff --git a/packages/backend/src/remote/activitypub/db-resolver.ts b/packages/backend/src/remote/activitypub/db-resolver.ts
index 52626f399..d8aa3b706 100644
--- a/packages/backend/src/remote/activitypub/db-resolver.ts
+++ b/packages/backend/src/remote/activitypub/db-resolver.ts
@@ -17,7 +17,7 @@ import { Cache } from "@/misc/cache.js";
 import { uriPersonCache, userByIdCache } from "@/services/user-cache.js";
 import type { IObject } from "./type.js";
 import { getApId } from "./type.js";
-import { resolvePerson } from "./models/person.js";
+import { resolvePerson, updatePerson } from "./models/person.js";
 import {redisClient, subscriber} from "@/db/redis.js";
 
 const publicKeyCache = new Cache<UserPublickey | null>("publicKey", 60 * 30);
@@ -152,7 +152,7 @@ export default class DbResolver {
 	 */
 	public async getAuthUserFromKeyId(keyId: string): Promise<{
 		user: CacheableRemoteUser;
-		key: UserPublickey;
+		key: UserPublickey | null;
 	} | null> {
 		const key = await publicKeyCache.fetch(
 			keyId,
@@ -204,6 +204,15 @@ export default class DbResolver {
 			key,
 		};
 	}
+
+	public async refetchPublicKeyForApId(user: CacheableRemoteUser): Promise<UserPublickey | null> {
+		await updatePerson(user.uri!, undefined, undefined, user);
+		let key = await UserPublickeys.findOneBy({ userId: user.id });
+		if (key != null) {
+			await publicKeyByUserIdCache.set(user.id, key);
+		}
+		return key;
+	}
 }
 
 subscriber.on("message", async (_, data) => {