diff --git a/packages/backend/src/core/FanoutTimelineEndpointService.ts b/packages/backend/src/core/FanoutTimelineEndpointService.ts
index c3c7ead201..d6d409f8f1 100644
--- a/packages/backend/src/core/FanoutTimelineEndpointService.ts
+++ b/packages/backend/src/core/FanoutTimelineEndpointService.ts
@@ -231,7 +231,7 @@ export class FanoutTimelineEndpointService {
 			Promise.all(ps.redisTimelines.map((tl, i) => {
 				// 有効なソースかつ結果が空だった場合のみダミーを入れる
 				if (redisResult[i] && redisResult[i].length === 0) {
-					return this.fanoutTimelineService.injectDummy(tl, dummyId);
+					return this.fanoutTimelineService.injectDummyIfEmpty(tl, dummyId);
 				}
 				return Promise.resolve();
 			}));
diff --git a/packages/backend/src/core/FanoutTimelineService.ts b/packages/backend/src/core/FanoutTimelineService.ts
index 619fd7a89d..f6f6ba8f6c 100644
--- a/packages/backend/src/core/FanoutTimelineService.ts
+++ b/packages/backend/src/core/FanoutTimelineService.ts
@@ -109,10 +109,20 @@ export class FanoutTimelineService {
 	}
 
 	@bindThis
-	public injectDummy(tl: FanoutTimelineName, id: string) {
+	injectDummy(tl: FanoutTimelineName, id: string) {
 		return this.redisForTimelines.lpush('list:' + tl, id);
 	}
 
+	@bindThis
+	public injectDummyIfEmpty(tl: FanoutTimelineName, id: string): Promise<boolean> {
+		return this.redisForTimelines.eval(
+			'if redis.call("LLEN", KEYS[1]) == 0 then redis.call("LPUSH", KEYS[1], ARGV[1]) return 1 else return 0 end',
+			1,
+			'list:' + tl,
+			id,
+		).then(res => res === 1);
+	}
+
 	@bindThis
 	public purge(name: FanoutTimelineName) {
 		return this.redisForTimelines.del('list:' + name);
diff --git a/packages/backend/test/unit/FanoutTimelineEndpointService.ts b/packages/backend/test/unit/FanoutTimelineEndpointService.ts
index 1dc605648f..06a9b923b1 100644
--- a/packages/backend/test/unit/FanoutTimelineEndpointService.ts
+++ b/packages/backend/test/unit/FanoutTimelineEndpointService.ts
@@ -68,6 +68,7 @@ describe('FanoutTimelineEndpointService', () => {
 			.useValue({
 				getMulti: jest.fn(),
 				injectDummy: jest.fn(),
+				injectDummyIfEmpty: jest.fn(),
 			})
 			.compile();
 
@@ -266,9 +267,9 @@ describe('FanoutTimelineEndpointService', () => {
 
 		expect(result).toEqual([]);
 		// Should have tried to inject dummy ID for both empty timelines
-		expect(fanoutTimelineService.injectDummy).toHaveBeenCalledTimes(2);
-		expect(fanoutTimelineService.injectDummy).toHaveBeenCalledWith(`homeTimeline:${alice.id}`, expect.any(String));
-		expect(fanoutTimelineService.injectDummy).toHaveBeenCalledWith('localTimeline', expect.any(String));
+		expect(fanoutTimelineService.injectDummyIfEmpty).toHaveBeenCalledTimes(2);
+		expect(fanoutTimelineService.injectDummyIfEmpty).toHaveBeenCalledWith(`homeTimeline:${alice.id}`, expect.any(String));
+		expect(fanoutTimelineService.injectDummyIfEmpty).toHaveBeenCalledWith('localTimeline', expect.any(String));
 	});
 
 	// Test for behavior when dummy ID exists
diff --git a/packages/backend/test/unit/FanoutTimelineService.ts b/packages/backend/test/unit/FanoutTimelineService.ts
new file mode 100644
index 0000000000..4d14400ff6
--- /dev/null
+++ b/packages/backend/test/unit/FanoutTimelineService.ts
@@ -0,0 +1,85 @@
+/*
+ * SPDX-FileCopyrightText: syuilo and misskey-project
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+import { describe, jest, test, expect, afterEach, beforeAll, afterAll } from '@jest/globals';
+import { Test, TestingModule } from '@nestjs/testing';
+import * as Redis from 'ioredis';
+import { FanoutTimelineService } from '@/core/FanoutTimelineService.js';
+import { IdService } from '@/core/IdService.js';
+import { DI } from '@/di-symbols.js';
+
+describe('FanoutTimelineService', () => {
+	let app: TestingModule;
+	let service: FanoutTimelineService;
+	let redisForTimelines: jest.Mocked<Redis.Redis>;
+	let idService: IdService;
+
+	beforeAll(async () => {
+		app = await Test.createTestingModule({
+			providers: [
+				FanoutTimelineService,
+				{
+					provide: IdService,
+					useValue: {
+						parse: jest.fn(),
+						gen: jest.fn(),
+					},
+				},
+				{
+					provide: DI.redisForTimelines,
+					useValue: {
+						eval: jest.fn(),
+						lpush: jest.fn(),
+						lrange: jest.fn(),
+						del: jest.fn(),
+						pipeline: jest.fn(() => ({
+							lpush: jest.fn(),
+							ltrim: jest.fn(),
+							lrange: jest.fn(),
+							exec: jest.fn(),
+						})),
+					},
+				},
+			],
+		}).compile();
+
+		app.enableShutdownHooks();
+
+		service = app.get<FanoutTimelineService>(FanoutTimelineService);
+		redisForTimelines = app.get(DI.redisForTimelines);
+		idService = app.get<IdService>(IdService);
+	});
+
+	afterAll(async () => {
+		await app.close();
+	});
+
+	afterEach(async () => {
+		jest.clearAllMocks();
+	});
+
+	test('injectDummyIfEmpty should call Redis EVAL with correct script', async () => {
+		redisForTimelines.eval.mockResolvedValue(1);
+
+		const result = await service.injectDummyIfEmpty('homeTimeline:123', 'dummyId');
+
+		expect(redisForTimelines.eval).toHaveBeenCalledWith(
+			expect.stringContaining('if redis.call("LLEN", KEYS[1]) == 0 then'),
+			1,
+			'list:homeTimeline:123',
+			'dummyId',
+		);
+		expect(result).toBe(true);
+	});
+
+	test('injectDummyIfEmpty should return false if list is not empty', async () => {
+		redisForTimelines.eval.mockResolvedValue(0);
+
+		const result = await service.injectDummyIfEmpty('homeTimeline:123', 'dummyId');
+
+		expect(redisForTimelines.eval).toHaveBeenCalled();
+		expect(result).toBe(false);
+	});
+});