diff --git a/packages/backend/src/misc/before-shutdown.ts b/packages/backend/src/misc/before-shutdown.ts
index 08204183..c259038d 100644
--- a/packages/backend/src/misc/before-shutdown.ts
+++ b/packages/backend/src/misc/before-shutdown.ts
@@ -6,6 +6,7 @@
  * @callback BeforeShutdownListener
  * @param {string} [signalOrEvent] The exit signal or event name received on the process.
  */
+type BeforeShutdownListener = (signalOrEvent: string) => Promise<void>;
 
 /**
  * System signals the app will listen to initiate shutdown.
@@ -24,7 +25,7 @@ const SHUTDOWN_TIMEOUT = 15000;
  * down the process.
  * @type {BeforeShutdownListener[]}
  */
-const shutdownListeners: ((signalOrEvent: string) => void)[] = [];
+const shutdownListeners: BeforeShutdownListener[] = [];
 
 /**
  * Listen for signals and execute given `fn` function once.
@@ -89,7 +90,9 @@ async function shutdownHandler(signalOrEvent: string) {
  * @param {BeforeShutdownListener} listener The shutdown listener to register.
  * @returns {BeforeShutdownListener} Echoes back the supplied `listener`.
  */
-export function beforeShutdown(listener: () => void) {
+export function beforeShutdown(
+	listener: BeforeShutdownListener,
+): BeforeShutdownListener {
 	shutdownListeners.push(listener);
 	return listener;
 }
diff --git a/packages/backend/src/services/chart/index.ts b/packages/backend/src/services/chart/index.ts
index 4b8a9278..b084941f 100644
--- a/packages/backend/src/services/chart/index.ts
+++ b/packages/backend/src/services/chart/index.ts
@@ -6,4 +6,4 @@ export const activeUsersChart = new ActiveUsersChart();
 // 20分おきにメモリ情報をDBに書き込み
 setInterval(() => activeUsersChart.save(), 1000 * 60 * 20);
 
-beforeShutdown(async () => await new Promise(() => activeUsersChart.save()));
+beforeShutdown(() => activeUsersChart.save());