diff --git a/Sora/Sora.cs b/Sora/Sora.cs
index 8f384ae..f1e2cd1 100644
--- a/Sora/Sora.cs
+++ b/Sora/Sora.cs
@@ -868,9 +868,9 @@ public void GetStats(Action<string> onGetStats)
     }
 
     public (bool, string) GetTrackId(uint trackId) {
-        bool found = false;
-        string trackId_ = get_track_id(trackId, ref found);
-        return (found, trackId_);
+        IntPtr p = get_track_id(trackId);
+        bool found = p != IntPtr.Zero;
+        return (found, found ? Marshal.PtrToStringAnsi(p) : "");
     }
 
     /// <summary>
@@ -1102,9 +1102,8 @@ public string ConnectedSignalingURL
     private static extern void sora_get_selected_signaling_url(IntPtr p, [Out] byte[] buf, int size);
     [DllImport(DllName)]
     private static extern void sora_get_connected_signaling_url(IntPtr p, [Out] byte[] buf, int size);
-    [DllImport(DllName)]
-    [return: MarshalAs(UnmanagedType.LPStr)]
-    private static extern string get_track_id(uint track_id, [MarshalAs(UnmanagedType.I1)] ref bool found);
+    [DllImport(DllName, CharSet=CharSet.Ansi)]
+    private static extern IntPtr get_track_id(uint track_id, [MarshalAs(UnmanagedType.I1)] ref bool found);
 
     public class AudioOutputHelper : IDisposable
     {
diff --git a/src/unity.cpp b/src/unity.cpp
index 8afa332..269f4ac 100644
--- a/src/unity.cpp
+++ b/src/unity.cpp
@@ -310,14 +310,11 @@ void sora_audio_output_helper_set_handsfree(void* p, unity_bool_t enabled) {
   helper->SetHandsfree(enabled != 0);
 }
 
-UNITY_INTERFACE_EXPORT const char* get_track_id(ptrid_t track_id,
-                                                unity_bool_t* found) {
+UNITY_INTERFACE_EXPORT const char* get_track_id(ptrid_t track_id) {
   void* p = sora_unity_sdk::IdPointer::Instance().Lookup(track_id);
-  *found = p != nullptr ? 1 : 0;
-
-  // return *found ? ((sora_unity_sdk::UnityRenderer::Sink*)p)->GetTrackId().c_str()
-  return *found ? ((sora_unity_sdk::UnityRenderer::Sink*)p)->GetTrackIdC()
-                : nullptr;
+  return p != nullptr
+             ? ((sora_unity_sdk::UnityRenderer::Sink*)p)->GetTrackId().c_str()
+             : nullptr;
 }
 
 // iOS の場合は static link で名前が被る可能性があるので、別の名前にしておく
diff --git a/src/unity.h b/src/unity.h
index b568fd6..4405d8e 100644
--- a/src/unity.h
+++ b/src/unity.h
@@ -126,8 +126,7 @@ sora_audio_output_helper_is_handsfree(void* p);
 UNITY_INTERFACE_EXPORT void sora_audio_output_helper_set_handsfree(
     void* p,
     unity_bool_t enabled);
-UNITY_INTERFACE_EXPORT const char* get_track_id(ptrid_t track_id,
-                                                unity_bool_t* found);
+UNITY_INTERFACE_EXPORT const char* get_track_id(ptrid_t track_id);
 
 #ifdef __cplusplus
 }
diff --git a/src/unity_renderer.cpp b/src/unity_renderer.cpp
index be711e8..936a620 100644
--- a/src/unity_renderer.cpp
+++ b/src/unity_renderer.cpp
@@ -17,10 +17,6 @@ UnityRenderer::Sink::Sink(webrtc::VideoTrackInterface* track)
   updating_ = false;
   ptrid_ = IdPointer::Instance().Register(this);
   track_->AddOrUpdateSink(this, rtc::VideoSinkWants());
-
-  size_t size = strlen(track_id_.c_str());
-  track_id_c_ = new char[size + 1];
-  strcpy(track_id_c_, track_id_.c_str());
 }
 UnityRenderer::Sink::~Sink() {
   RTC_LOG(LS_INFO) << "[" << (void*)this << "] Sink::~Sink";
@@ -41,9 +37,6 @@ ptrid_t UnityRenderer::Sink::GetSinkID() const {
 const std::string& UnityRenderer::Sink::GetTrackId() const {
   return track_id_;
 }
-char* UnityRenderer::Sink::GetTrackIdC() const {
-  return track_id_c_;
-}
 
 void UnityRenderer::Sink::SetTrack(webrtc::VideoTrackInterface* track) {
   track_->RemoveSink(this);
diff --git a/src/unity_renderer.h b/src/unity_renderer.h
index c1a50d2..f388397 100644
--- a/src/unity_renderer.h
+++ b/src/unity_renderer.h
@@ -32,7 +32,6 @@ class UnityRenderer {
     ~Sink();
     ptrid_t GetSinkID() const;
     const std::string& GetTrackId() const;
-    char* GetTrackIdC() const;
     void SetTrack(webrtc::VideoTrackInterface* track);
 
    private: