diff --git a/ChangeLog b/ChangeLog
index 448b8ad811..00a88d29a7 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,7 @@
+Sat Aug 18 17:40:11 2007  Nobuyoshi Nakada  <nobu@ruby-lang.org>
+
+	* thread.c (is_ruby_native_thread): check properly.  [ruby-dev:31166]
+
 Sat Aug 18 16:44:15 2007  Koichi Sasada  <ko1@atdot.net>
 
 	* insnhelper.ci (vm_call_bmethod), 
diff --git a/thread.c b/thread.c
index 69f3b29e91..aec4340d5e 100644
--- a/thread.c
+++ b/thread.c
@@ -3002,5 +3002,8 @@ Init_Thread(void)
 int
 is_ruby_native_thread(void)
 {
-    return Qtrue;
+    rb_thread_t *rb_thread_check_ptr(rb_thread_t *ptr);
+    rb_thread_t *th = ruby_thread_from_native();
+
+    return th ? Qtrue : Qfalse;
 }
diff --git a/thread_pthread.ci b/thread_pthread.ci
index d624e98487..b8b609145e 100644
--- a/thread_pthread.ci
+++ b/thread_pthread.ci
@@ -72,16 +72,34 @@ static void remove_signal_thread_list(rb_thread_t *th);
 
 static rb_thread_lock_t signal_thread_list_lock;
 
+static pthread_key_t ruby_native_thread_key;
+
 static void
 null_func()
 {
     /* null */
 }
 
-static void
-Init_native_thread()
+static rb_thread_t *
+ruby_thread_from_native(void)
 {
-    GET_THREAD()->thread_id = pthread_self();
+    return pthread_getspecific(ruby_native_thread_key);
+}
+
+static int
+ruby_thread_set_native(rb_thread_t *th)
+{
+    return pthread_setspecific(ruby_native_thread_key, th) == 0;
+}
+
+static void
+Init_native_thread(void)
+{
+    rb_thread_t *th = GET_THREAD();
+
+    pthread_key_create(&ruby_native_thread_key, NULL);
+    th->thread_id = pthread_self();
+    ruby_thread_set_native(th);
     native_mutex_initialize(&signal_thread_list_lock);
     posix_signal(SIGVTALRM, null_func);
 }
diff --git a/thread_win32.ci b/thread_win32.ci
index bfb7f681c7..3bdb2e4276 100644
--- a/thread_win32.ci
+++ b/thread_win32.ci
@@ -20,10 +20,26 @@
 #define native_thread_yield() Sleep(0)
 #define remove_signal_thread_list(th)
 
+static volatile DWORD ruby_native_thread_key = TLS_OUT_OF_INDEXES;
+
+static rb_thread_t *
+ruby_thread_from_native(void)
+{
+    return TlsGetValue(ruby_native_thread_key);
+}
+
+static int
+ruby_thread_set_native(rb_thread_t *th)
+{
+    return TlsSetValue(ruby_native_thread_key, th);
+}
+
 static void
-Init_native_thread()
+Init_native_thread(void)
 {
     rb_thread_t *th = GET_THREAD();
+
+    ruby_native_thread_key = TlsAlloc();
     DuplicateHandle(GetCurrentProcess(),
 		    GetCurrentThread(),
 		    GetCurrentProcess(),