diff --git a/CHANGES.md b/CHANGES.md
index 30fd6c84..b96a545e 100644
--- a/CHANGES.md
+++ b/CHANGES.md
@@ -11,6 +11,9 @@
 
 ## develop
 
+- [CHANGE] シグナリングオプションの `multistream` は false の時のみレガシーストリームを使用する
+  - レガシーストリームを使用したい場合は `multistream: false` の指定が必須となる
+  - @tnamao
 - [CHANGE] stopAudioTrack と stopVideoTrack を非推奨にする
   - 代わりに名前を変えただけの removeAudioTrack と removeVideoTrack を用意する
   - @voluntas
diff --git a/examples/check.html b/examples/check.html
index 7ececf39..716ce47e 100644
--- a/examples/check.html
+++ b/examples/check.html
@@ -21,7 +21,7 @@ <h1>リリース前の動作確認用サンプル</h1>
       <small>※ クエリストリングの signalingUrl パラメータでも指定可能</small>
       <hr />
 
-      <h2><u>チャネル ID: sora-js-sdk:check:singlestream</u></h2>
+      <h2><u>チャネル ID: sora-js-sdk:check:legacystream</u></h2>
 
       要確認ブラウザ:
       <ul>
@@ -30,14 +30,14 @@ <h2><u>チャネル ID: sora-js-sdk:check:singlestream</u></h2>
         <li>Firefox: Linux or macOS or Windows</li>
       </ul>
 
-      <button onclick="startSinglestreamCheck()">接続</button>
-      <button onclick="stopSinglestreamCheck()">切断</button>
+      <button onclick="startLegacystreamCheck()">接続</button>
+      <button onclick="stopLegacystreamCheck()">切断</button>
 
       <h4>sendonly, Opus</h4>
-      <div id="singlestream-sendonly-videos" class="singlestream"></div>
+      <div id="legacystream-sendonly-videos" class="legacystream"></div>
 
       <h4>recvonly</h4>
-      <div id="singlestream-recvonly-videos" class="singlestream"></div>
+      <div id="legacystream-recvonly-videos" class="legacystream"></div>
 
       <h2><u>チャネル ID: sora-js-sdk:check:multistream</u></h2>
 
@@ -183,23 +183,23 @@ <h4>multistream, E2EE, recvonly</h4>
           document.querySelectorAll('.multistream').forEach(x => x.innerHTML = null);
       }
 
-      let singlestreamCheckConnections = [];
-      async function startSinglestreamCheck() {
-          stopSinglestreamCheck();
+      let legacystreamCheckConnections = [];
+      async function startLegacystreamCheck() {
+          stopLegacystreamCheck();
 
-          const channelId = 'sora-js-sdk:check:singlestream';
-          singlestreamCheckConnections = [
-              await connect(channelId, 'singlestream-sendonly-videos', 'sendonly',
+          const channelId = 'sora-js-sdk:check:legacystream';
+          legacystreamCheckConnections = [
+              await connect(channelId, 'legacystream-sendonly-videos', 'sendonly',
                             {multistream: false}),
-              await connect(channelId, 'singlestream-recvonly-videos', 'recvonly',
+              await connect(channelId, 'legacystream-recvonly-videos', 'recvonly',
                             {multistream: false}),
           ];
       }
 
-      async function stopSinglestreamCheck() {
-          singlestreamCheckConnections.forEach(x => x.disconnect());
-          singlestreamCheckConnections = [];
-          document.querySelectorAll('.singlestream').forEach(x => x.innerHTML = null);
+      async function stopLegacystreamCheck() {
+          legacystreamCheckConnections.forEach(x => x.disconnect());
+          legacystreamCheckConnections = [];
+          document.querySelectorAll('.legacystream').forEach(x => x.innerHTML = null);
       }
 
       function initLyra() {
diff --git a/packages/sdk/src/publisher.ts b/packages/sdk/src/publisher.ts
index a81a204b..7333ef3d 100644
--- a/packages/sdk/src/publisher.ts
+++ b/packages/sdk/src/publisher.ts
@@ -19,9 +19,10 @@ export default class ConnectionPublisher extends ConnectionBase {
    * @public
    */
   async connect(stream: MediaStream): Promise<MediaStream> {
-    if (this.options.multistream) {
+    // options.multistream が明示的に false を指定した時だけレガシーストリームにする
+    if (this.options.multistream === false) {
       await Promise.race([
-        this.multiStream(stream).finally(() => {
+        this.legacyStream(stream).finally(() => {
           this.clearConnectionTimeout()
           this.clearMonitorSignalingWebSocketEvent()
         }),
@@ -30,7 +31,7 @@ export default class ConnectionPublisher extends ConnectionBase {
       ])
     } else {
       await Promise.race([
-        this.singleStream(stream).finally(() => {
+        this.multiStream(stream).finally(() => {
           this.clearConnectionTimeout()
           this.clearMonitorSignalingWebSocketEvent()
         }),
@@ -44,11 +45,11 @@ export default class ConnectionPublisher extends ConnectionBase {
   }
 
   /**
-   * シングルストリームで Sora へ接続するメソッド
+   * レガシーストリームで Sora へ接続するメソッド
    *
    * @param stream - メディアストリーム
    */
-  private async singleStream(stream: MediaStream): Promise<MediaStream> {
+  private async legacyStream(stream: MediaStream): Promise<MediaStream> {
     await this.disconnect()
     this.setupE2EE()
     const ws = await this.getSignalingWebSocket(this.signalingUrlCandidates)
diff --git a/packages/sdk/src/sora.ts b/packages/sdk/src/sora.ts
index 68b902b4..499ab0ea 100644
--- a/packages/sdk/src/sora.ts
+++ b/packages/sdk/src/sora.ts
@@ -82,14 +82,12 @@ class SoraConnection {
     metadata: JSONType = null,
     options: ConnectionOptions = { audio: true, video: true },
   ): ConnectionPublisher {
-    // sendrecv の場合、multistream に初期値を指定する
-    const sendrecvOptions: ConnectionOptions = Object.assign({ multistream: true }, options)
     return new ConnectionPublisher(
       this.signalingUrlCandidates,
       'sendrecv',
       channelId,
       metadata,
-      sendrecvOptions,
+      options,
       this.debug,
     )
   }
diff --git a/packages/sdk/src/subscriber.ts b/packages/sdk/src/subscriber.ts
index f1dfaf7e..68b21f44 100644
--- a/packages/sdk/src/subscriber.ts
+++ b/packages/sdk/src/subscriber.ts
@@ -17,9 +17,10 @@ export default class ConnectionSubscriber extends ConnectionBase {
    */
   // biome-ignore lint/suspicious/noConfusingVoidType: stream が <MediaStream | void> なのでどうしようもない
   async connect(): Promise<MediaStream | void> {
-    if (this.options.multistream) {
-      await Promise.race([
-        this.multiStream().finally(() => {
+    // options.multistream が明示的に false を指定した時だけレガシーストリームにする
+    if (this.options.multistream === false) {
+      const stream = await Promise.race([
+        this.legacyStream().finally(() => {
           this.clearConnectionTimeout()
           this.clearMonitorSignalingWebSocketEvent()
         }),
@@ -28,10 +29,10 @@ export default class ConnectionSubscriber extends ConnectionBase {
       ])
       this.monitorWebSocketEvent()
       this.monitorPeerConnectionState()
-      return
+      return stream
     }
-    const stream = await Promise.race([
-      this.singleStream().finally(() => {
+    await Promise.race([
+      this.multiStream().finally(() => {
         this.clearConnectionTimeout()
         this.clearMonitorSignalingWebSocketEvent()
       }),
@@ -40,13 +41,12 @@ export default class ConnectionSubscriber extends ConnectionBase {
     ])
     this.monitorWebSocketEvent()
     this.monitorPeerConnectionState()
-    return stream
   }
 
   /**
-   * シングルストリームで Sora へ接続するメソッド
+   * レガシーストリームで Sora へ接続するメソッド
    */
-  private async singleStream(): Promise<MediaStream> {
+  private async legacyStream(): Promise<MediaStream> {
     await this.disconnect()
     this.setupE2EE()
     const ws = await this.getSignalingWebSocket(this.signalingUrlCandidates)
diff --git a/packages/sdk/src/utils.ts b/packages/sdk/src/utils.ts
index 7809ab03..7b70aa4a 100644
--- a/packages/sdk/src/utils.ts
+++ b/packages/sdk/src/utils.ts
@@ -146,7 +146,8 @@ export function createSignalingMessage(
     video: true,
   }
   // role: sendrecv で multistream: false の場合は例外を発生させる
-  if (role === 'sendrecv' && options.multistream !== true) {
+  // options.multistream === undefined は multistream として扱う
+  if (role === 'sendrecv' && options.multistream === false) {
     throw new Error(
       "Failed to parse options. Options multistream must be true when connecting using 'sendrecv'",
     )
diff --git a/packages/sdk/tests/utils.test.ts b/packages/sdk/tests/utils.test.ts
index cd7a4772..34352ef9 100644
--- a/packages/sdk/tests/utils.test.ts
+++ b/packages/sdk/tests/utils.test.ts
@@ -1,6 +1,6 @@
-import { test, expect } from 'vitest'
-import { createSignalingMessage } from '../src/utils'
+import { expect, test } from 'vitest'
 import { type AudioCodecType, DataChannelDirection, VideoCodecType } from '../src/types'
+import { createSignalingMessage } from '../src/utils'
 
 const channelId = '7N3fsMHob'
 const metadata = 'PG9A6RXgYqiqWKOVO'
@@ -47,6 +47,15 @@ test('createSignalingMessage simple sendrecv', () => {
   ).toEqual(expectedMessage)
 })
 
+test('createSignalingMessage sendrecv and undefined multistream', () => {
+  const expectedMessage = Object.assign({}, baseExpectedMessage, {
+    role: 'sendrecv',
+  })
+  expect(createSignalingMessage(sdp, 'sendrecv', channelId, undefined, {}, false)).toEqual(
+    expectedMessage,
+  )
+})
+
 test('createSignalingMessage invalid role', () => {
   expect(() => {
     createSignalingMessage(sdp, 'test', channelId, metadata, {}, false)
@@ -55,7 +64,7 @@ test('createSignalingMessage invalid role', () => {
 
 test('createSignalingMessage sendrecv and multistream: false', () => {
   expect(() => {
-    createSignalingMessage(sdp, 'sendrecv', channelId, metadata, {}, false)
+    createSignalingMessage(sdp, 'sendrecv', channelId, metadata, { multistream: false }, false)
   }).toThrow(
     Error(
       "Failed to parse options. Options multistream must be true when connecting using 'sendrecv'",
@@ -140,6 +149,14 @@ test('createSignalingMessage multistream: true', () => {
   )
 })
 
+test('createSignalingMessage undefined multistream', () => {
+  const options = {}
+  const expectedMessage = Object.assign({}, baseExpectedMessage, {})
+  expect(createSignalingMessage(sdp, 'sendonly', channelId, undefined, options, false)).toEqual(
+    expectedMessage,
+  )
+})
+
 test('createSignalingMessage multistream: false', () => {
   const options = { multistream: false }
   const expectedMessage = Object.assign({}, baseExpectedMessage, {