diff --git a/packages/backend/src/core/activitypub/ApDbResolverService.ts b/packages/backend/src/core/activitypub/ApDbResolverService.ts
index 87e3b7ed5f..a5259b1c4a 100644
--- a/packages/backend/src/core/activitypub/ApDbResolverService.ts
+++ b/packages/backend/src/core/activitypub/ApDbResolverService.ts
@@ -10,11 +10,13 @@ import type { Config } from '@/config.js';
 import { MemoryKVCache } from '@/misc/cache.js';
 import type { MiUserPublickey } from '@/models/UserPublickey.js';
 import { CacheService } from '@/core/CacheService.js';
+import { TimeService } from '@/global/TimeService.js';
 import { UtilityService } from '@/core/UtilityService.js';
 import type { MiNote } from '@/models/Note.js';
 import { bindThis } from '@/decorators.js';
 import type { MiLocalUser, MiRemoteUser } from '@/models/User.js';
 import { ApLoggerService } from '@/core/activitypub/ApLoggerService.js';
+import { IdService } from '@/core/IdService.js';
 import { getApId } from './type.js';
 import { ApPersonService } from './models/ApPersonService.js';
 import type { IObject } from './type.js';
@@ -40,6 +42,8 @@ export class ApDbResolverService implements OnApplicationShutdown {
 		private apPersonService: ApPersonService,
 		private apLoggerService: ApLoggerService,
 		private utilityService: UtilityService,
+		private readonly idService: IdService,
+		private readonly timeService: TimeService,
 	) {
 		// Caches moved to ApPersonService to avoid circular dependency
 	}
@@ -125,20 +129,36 @@ export class ApDbResolverService implements OnApplicationShutdown {
 	 */
 	@bindThis
 	public async refetchPublicKeyForApId(user: MiRemoteUser): Promise<MiUserPublickey | null> {
+		const oldKey = await this.apPersonService.findPublicKeyByUserId(user.id);
+
+		// Don't re-fetch if we've updated the user recently
+		const maxUpdatedTime = this.timeService.now - (1000 * 60 * 60); // 1 hour
+		if ((user.lastFetchedAt && user.lastFetchedAt.valueOf() > maxUpdatedTime) ||
+			(user.updatedAt && user.updatedAt.valueOf() > maxUpdatedTime) ||
+			this.idService.parse(user.id).date.valueOf() > maxUpdatedTime
+		) {
+			this.apLoggerService.logger.debug(`Not updating public key for user ${user.id} (${user.uri}): already checked recently`);
+			return oldKey;
+		}
+
 		this.apLoggerService.logger.debug(`Updating public key for user ${user.id} (${user.uri})`);
 
-		const oldKey = await this.apPersonService.findPublicKeyByUserId(user.id);
+		// updatePerson will update the public key cache if there's any changes.
 		await this.apPersonService.updatePerson(user.uri);
 		const newKey = await this.apPersonService.findPublicKeyByUserId(user.id);
 
-		if (newKey) {
-			if (oldKey && newKey.keyPem === oldKey.keyPem) {
+		if (newKey && oldKey) {
+			if (newKey.keyPem === oldKey.keyPem) {
 				this.apLoggerService.logger.debug(`Public key is up-to-date for user ${user.id} (${user.uri})`);
 			} else {
 				this.apLoggerService.logger.info(`Updated public key for user ${user.id} (${user.uri})`);
 			}
+		} else if (newKey) {
+			this.apLoggerService.logger.info(`Registered public key for user ${user.id} (${user.uri})`);
+		} else if (oldKey) {
+			this.apLoggerService.logger.info(`Deleted public key for user ${user.id} (${user.uri})`);
 		} else {
-			this.apLoggerService.logger.warn(`Failed to update public key for user ${user.id} (${user.uri})`);
+			this.apLoggerService.logger.warn(`Could not find any public key for user ${user.id} (${user.uri})`);
 		}
 
 		return newKey ?? oldKey;
diff --git a/packages/backend/src/queue/processors/InboxProcessorService.ts b/packages/backend/src/queue/processors/InboxProcessorService.ts
index 4efbb088c6..13b2885263 100644
--- a/packages/backend/src/queue/processors/InboxProcessorService.ts
+++ b/packages/backend/src/queue/processors/InboxProcessorService.ts
@@ -164,7 +164,12 @@ export class InboxProcessorService implements OnApplicationShutdown {
 
 		// publicKey がなくても終了
 		if (authUser.key == null) {
-			throw new Bull.UnrecoverableError(`skip: failed to resolve user publicKey ${actorId}`);
+			// See if a key has become available since we fetched the actor
+			authUser.key = await this.apDbResolverService.refetchPublicKeyForApId(authUser.user);
+			if (authUser.key == null) {
+				// If it's still missing, then give up
+				throw new Bull.UnrecoverableError(`skip: failed to resolve user publicKey ${actorId}`);
+			}
 		}
 
 		// HTTP-Signatureの検証