diff --git a/src/features/core/internal/Voice/internal/Voice.class.spec.ts b/src/features/core/internal/Voice/internal/Voice.class.spec.ts
index e8e06751..42d89403 100644
--- a/src/features/core/internal/Voice/internal/Voice.class.spec.ts
+++ b/src/features/core/internal/Voice/internal/Voice.class.spec.ts
@@ -230,7 +230,7 @@ describe('Voice', () => {
         .mockReturnValueOnce('unknown' as any);
 
       await expect(voice.join({ interaction })).rejects.toThrowError(
-        'Unknown joinable type.'
+        'unknown is unexpected value. Should have been never.'
       );
     });
   });
diff --git a/src/features/core/internal/Voice/internal/Voice.class.ts b/src/features/core/internal/Voice/internal/Voice.class.ts
index 49710b4e..b9f902ea 100644
--- a/src/features/core/internal/Voice/internal/Voice.class.ts
+++ b/src/features/core/internal/Voice/internal/Voice.class.ts
@@ -111,11 +111,9 @@ export class Voice {
 
       default:
         assertNever(joinable);
-        // HACK: ここでrejectするとSwitch外のカバレッジがuncoveredになる
-        break;
     }
-
-    return Promise.reject(new LogicException('Unknown joinable type.'));
+    /* c8 ignore next */
+    return;
   }
 
   public async leave({
diff --git a/src/features/core/internal/Voice/internal/funcs/getJoinableStateStatus/internal/getJoinableStateStatus.func.ts b/src/features/core/internal/Voice/internal/funcs/getJoinableStateStatus/internal/getJoinableStateStatus.func.ts
index 84ffb332..491add4a 100644
--- a/src/features/core/internal/Voice/internal/funcs/getJoinableStateStatus/internal/getJoinableStateStatus.func.ts
+++ b/src/features/core/internal/Voice/internal/funcs/getJoinableStateStatus/internal/getJoinableStateStatus.func.ts
@@ -10,29 +10,28 @@ export const getJoinableStateStatus = ({
 }: {
   channel: VoiceBasedChannel | null;
 }): (typeof JOINABLE_STATE_STATUS)[keyof typeof JOINABLE_STATE_STATUS] => {
-  // getVoiceConnectionやgetVoiceConnectionsを利用せずに、自身がjoinしているかどうかを判定する
-  const client = channel?.client;
-  const guildId = channel?.guildId;
-  if (!client || !guildId) {
-    return JOINABLE_STATE_STATUS.NOT_FOUND;
-  }
-
-  const voiceState = (() => {
-    const guild = client.guilds.cache.get(guildId);
-    if (!guild) return undefined;
-    const voiceState = guild.voiceStates.cache.get(client.user.id);
-    return voiceState;
-  })();
-
   if (isNil(channel)) {
     return JOINABLE_STATE_STATUS.NOT_FOUND;
-  } else if (!channel.joinable) {
-    return JOINABLE_STATE_STATUS.NOT_JOINABLE;
-  } else if (!channel.viewable) {
-    return JOINABLE_STATE_STATUS.NOT_VIEWABLE;
-  } else if (!isNil(voiceState)) {
-    return JOINABLE_STATE_STATUS.ALREADY_JOINED;
   } else {
-    return JOINABLE_STATE_STATUS.JOINABLE;
+    // getVoiceConnectionやgetVoiceConnectionsを利用せずに、自身がjoinしているかどうかを判定する
+    const client = channel.client;
+    const guildId = channel.guildId;
+
+    const voiceState = (() => {
+      const guild = client.guilds.cache.get(guildId);
+      if (!guild) return undefined;
+      const voiceState = guild.voiceStates.cache.get(client.user.id);
+      return voiceState;
+    })();
+
+    if (!channel.joinable) {
+      return JOINABLE_STATE_STATUS.NOT_JOINABLE;
+    } else if (!channel.viewable) {
+      return JOINABLE_STATE_STATUS.NOT_VIEWABLE;
+    } else if (!!voiceState) {
+      return JOINABLE_STATE_STATUS.ALREADY_JOINED;
+    } else {
+      return JOINABLE_STATE_STATUS.JOINABLE;
+    }
   }
 };
diff --git a/src/features/core/internal/Voice/internal/funcs/getJoinableStateStatus/internal/getJoinableStateStatus.spec.ts b/src/features/core/internal/Voice/internal/funcs/getJoinableStateStatus/internal/getJoinableStateStatus.spec.ts
index e477231f..5b903fb5 100644
--- a/src/features/core/internal/Voice/internal/funcs/getJoinableStateStatus/internal/getJoinableStateStatus.spec.ts
+++ b/src/features/core/internal/Voice/internal/funcs/getJoinableStateStatus/internal/getJoinableStateStatus.spec.ts
@@ -1,17 +1,22 @@
 import type { VoiceBasedChannel } from '@/features/library/index.js';
 
 import { getJoinableStateStatus } from './getJoinableStateStatus.func.js';
+import { JOINABLE_STATE_STATUS } from './getJoinableStateStatus.constants.js';
 
 describe('getJoinableStateStatus', () => {
   it('should return NOT_FOUND if the channel is null', () => {
     const result = getJoinableStateStatus({ channel: null });
 
-    expect(result).toBe('NOT_FOUND');
+    expect(result).toBe(JOINABLE_STATE_STATUS.NOT_FOUND);
   });
 
   it('should return NOT_JOINABLE if the channel is not joinable', () => {
     const result = getJoinableStateStatus({
-      channel: { joinable: false } as VoiceBasedChannel,
+      channel: {
+        joinable: false,
+        client: { guilds: { cache: new Map() } },
+        guildId: 'guildId',
+      } as VoiceBasedChannel,
     });
 
     expect(result).toBe('NOT_JOINABLE');
@@ -19,7 +24,12 @@ describe('getJoinableStateStatus', () => {
 
   it('should return NOT_VIEWABLE if the channel is not viewable', () => {
     const result = getJoinableStateStatus({
-      channel: { joinable: true, viewable: false } as VoiceBasedChannel,
+      channel: {
+        joinable: true,
+        viewable: false,
+        client: { guilds: { cache: new Map() } },
+        guildId: 'guildId',
+      } as VoiceBasedChannel,
     });
 
     expect(result).toBe('NOT_VIEWABLE');
@@ -27,9 +37,43 @@ describe('getJoinableStateStatus', () => {
 
   it('should return JOINABLE if the channel is joinable and viewable', () => {
     const result = getJoinableStateStatus({
-      channel: { joinable: true, viewable: true } as VoiceBasedChannel,
+      channel: {
+        joinable: true,
+        viewable: true,
+        client: { guilds: { cache: new Map() } },
+        guildId: 'guildId',
+      } as VoiceBasedChannel,
     });
 
-    expect(result).toBe('JOINABLE');
+    expect(result).toBe(JOINABLE_STATE_STATUS.JOINABLE);
+  });
+
+  it('should return undefined if the voice state is not found', () => {
+    const result = getJoinableStateStatus({
+      channel: {
+        joinable: true,
+        viewable: true,
+        client: {
+          user: { id: 'userId' },
+          guilds: {
+            cache: new Map([
+              [
+                'guildId',
+                {
+                  voiceStates: {
+                    cache: new Map([
+                      ['userId', { voiceStates: { cache: new Map() } }],
+                    ]),
+                  },
+                },
+              ],
+            ]),
+          },
+        },
+        guildId: 'guildId',
+      } as unknown as VoiceBasedChannel,
+    });
+
+    expect(result).toBe(JOINABLE_STATE_STATUS.ALREADY_JOINED);
   });
 });