diff --git a/CHANGELOG.md b/CHANGELOG.md
index e133a32a9c..04ebcff386 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -7,7 +7,7 @@
 -
 
 ### Server
--
+- Fix: ID生成アルゴリズムにULIDを使用している場合に通知が約10秒遅延する問題を修正
 
 
 ## 2026.5.0
diff --git a/packages/backend/src/core/NotificationService.ts b/packages/backend/src/core/NotificationService.ts
index 310ffec7ce..670cecf01e 100644
--- a/packages/backend/src/core/NotificationService.ts
+++ b/packages/backend/src/core/NotificationService.ts
@@ -246,7 +246,8 @@ export class NotificationService implements OnApplicationShutdown {
 
 	private toXListId(id: string): string {
 		const { date, additional } = this.idService.parseFull(id);
-		return date.toString() + '-' + additional.toString();
+		// Redis Stream sequenceはunit64制約があるため、収まらない場合は下位64bitを取る
+		return date.toString() + '-' + BigInt.asUintN(64, additional).toString();
 	}
 
 	@bindThis
diff --git a/packages/backend/test/unit/misc/ulid.ts b/packages/backend/test/unit/misc/ulid.ts
index 4dbcc2b7e8..debca9bde9 100644
--- a/packages/backend/test/unit/misc/ulid.ts
+++ b/packages/backend/test/unit/misc/ulid.ts
@@ -38,4 +38,12 @@ describe('misc:ulid', () => {
 		// id[16] = Z
 		expect(() => parseUlidFull('01KPS7S300ABCDEFZ000000000')).not.toThrow();
 	});
+
+	test('parseUlidFull - additional exceeds uint64 max (all-Z randomness)', () => {
+		// All 16 random chars = 'Z' (Crockford max) → 80-bit value > uint64 max
+		const { additional } = parseUlidFull('01ARZ3NDEKZZZZZZZZZZZZZZZZ');
+		const uint64Max = 2n ** 64n - 1n;
+		expect(additional > uint64Max).toBe(true);
+		expect(BigInt.asUintN(64, additional) <= uint64Max).toBe(true);
+	});
 });