diff --git a/lib/includes/spdylay/spdylay.h b/lib/includes/spdylay/spdylay.h
index 056f58a..f2ed357 100644
--- a/lib/includes/spdylay/spdylay.h
+++ b/lib/includes/spdylay/spdylay.h
@@ -663,7 +663,8 @@ void* spdylay_session_get_stream_user_data(spdylay_session *session,
  * negative error codes:
  *
  * SPDYLAY_ERR_INVALID_ARGUMENT
- *     |pri| is invalid.
+ *     The |pri| is invalid; or the Associated-To-Stream-ID is
+ *     invalid.
  * SPDYLAY_ERR_NOMEM
  *     Out of memory.
  */
@@ -722,14 +723,15 @@ int spdylay_submit_response(spdylay_session *session,
  * data which is associated to the stream this frame will open.
  *
  * This function is low-level in a sense that the application code can
- * specify flags and assoc-stream-ID directly. For usual HTTP request,
- * spdylay_submit_request() is useful.
+ * specify flags and the Associated-To-Stream-ID directly. For usual
+ * HTTP request, spdylay_submit_request() is useful.
  *
  * This function returns 0 if it succeeds, or one of the following
  * negative error codes:
  *
  * SPDYLAY_ERR_INVALID_ARGUMENT
- *     |pri| is invalid.
+ *     The |pri| is invalid; or the Associated-To-Stream-ID is
+ *     invalid.
  * SPDYLAY_ERR_NOMEM
  *     Out of memory.
  */
diff --git a/lib/spdylay_session.c b/lib/spdylay_session.c
index 17a751c..04e3150 100644
--- a/lib/spdylay_session.c
+++ b/lib/spdylay_session.c
@@ -478,6 +478,45 @@ static int spdylay_predicate_stream_for_send(spdylay_stream *stream)
   }
 }
 
+/*
+ * This function checks SYN_STREAM frame |frame| can be sent at this
+ * time.
+ *
+ * This function returns 0 if it succeeds, or one of the following
+ * negative error codes:
+ *
+ * SPDYLAY_ERR_STREAM_CLOSED
+ *     The Associated-To-Stream is already closed or does not exist.
+ * SPDYLAY_ERR_STREAM_ID_NOT_AVAILABLE
+ *     Stream ID has reached maximum value. Therefore no stream ID is
+ *    available.
+ */
+static int spdylay_session_predicate_syn_stream_send
+(spdylay_session *session,
+ spdylay_syn_stream *frame)
+{
+  if(session->goaway_flags) {
+    /* When GOAWAY is sent or received, peer must not send new
+       SYN_STREAM. */
+    return SPDYLAY_ERR_SYN_STREAM_NOT_ALLOWED;
+  }
+  /* All 32bit signed stream IDs are spent. */
+  if(session->next_stream_id > INT32_MAX) {
+    return SPDYLAY_ERR_STREAM_ID_NOT_AVAILABLE;
+  }
+  if(frame->assoc_stream_id != 0) {
+    /* Check associated stream is active. */
+    /* We assume here that if frame->assoc_stream_id != 0,
+       session->server is always 1 and frame->assoc_stream_id is
+       odd. */
+    if(spdylay_session_get_stream(session, frame->assoc_stream_id) ==
+       NULL) {
+      return SPDYLAY_ERR_STREAM_CLOSED;
+    }
+  }
+  return 0;
+}
+
 /*
  * This function checks SYN_REPLY with the stream ID |stream_id| can
  * be sent at this time.
@@ -673,21 +712,16 @@ static int spdylay_session_predicate_data_send(spdylay_session *session,
 static ssize_t spdylay_session_prep_frame(spdylay_session *session,
                                           spdylay_outbound_item *item)
 {
-  /* TODO Get or validate stream ID here */
-  /* TODO Validate assoc_stream_id here */
   ssize_t framebuflen;
   switch(item->frame_type) {
   case SPDYLAY_SYN_STREAM: {
     int32_t stream_id;
     spdylay_syn_stream_aux_data *aux_data;
-    if(session->goaway_flags) {
-      /* When GOAWAY is sent or received, peer must not send new
-         SYN_STREAM. */
-      return SPDYLAY_ERR_SYN_STREAM_NOT_ALLOWED;
-    }
-    /* All 32bit signed stream IDs are spent. */
-    if(session->next_stream_id > INT32_MAX) {
-      return SPDYLAY_ERR_STREAM_ID_NOT_AVAILABLE;
+    int r;
+    r = spdylay_session_predicate_syn_stream_send(session,
+                                                  &item->frame->syn_stream);
+    if(r != 0) {
+      return r;
     }
     stream_id = session->next_stream_id;
 
diff --git a/lib/spdylay_submit.c b/lib/spdylay_submit.c
index 7671e3a..d99cf74 100644
--- a/lib/spdylay_submit.c
+++ b/lib/spdylay_submit.c
@@ -47,8 +47,12 @@ static int spdylay_submit_syn_stream_shared
   if(pri > spdylay_session_get_pri_lowest(session)) {
     return SPDYLAY_ERR_INVALID_ARGUMENT;
   }
-  if(session->server == 0) {
-    assoc_stream_id = 0;
+  if(assoc_stream_id != 0) {
+    if(session->server == 0) {
+      assoc_stream_id = 0;
+    } else if(spdylay_session_is_my_stream_id(session, assoc_stream_id)) {
+      return SPDYLAY_ERR_INVALID_ARGUMENT;
+    }
   }
   if(data_prd != NULL && data_prd->read_callback != NULL) {
     data_prd_copy = malloc(sizeof(spdylay_data_provider));
diff --git a/tests/spdylay_session_test.c b/tests/spdylay_session_test.c
index 8fc135a..98a5b88 100644
--- a/tests/spdylay_session_test.c
+++ b/tests/spdylay_session_test.c
@@ -730,6 +730,11 @@ void test_spdylay_submit_syn_stream()
   CU_ASSERT(1 == item->frame->syn_stream.assoc_stream_id);
   CU_ASSERT(3 == item->frame->syn_stream.pri);
 
+  /* Invalid assoc-stream-ID */
+  CU_ASSERT(SPDYLAY_ERR_INVALID_ARGUMENT ==
+            spdylay_submit_syn_stream(session, SPDYLAY_CTRL_FLAG_FIN, 2, 3,
+                                      nv, NULL));
+
   spdylay_session_del(session);
 }
 
@@ -1738,9 +1743,21 @@ void test_spdylay_session_on_ctrl_not_send()
   stream = spdylay_session_open_stream(session, 1,
                                        SPDYLAY_CTRL_FLAG_NONE,
                                        3, SPDYLAY_STREAM_OPENED, &user_data);
+  /* Check SYN_STREAM */
+  CU_ASSERT(0 == spdylay_submit_syn_stream(session, SPDYLAY_CTRL_FLAG_FIN, 3, 3,
+                                           nv, NULL));
+
+  user_data.ctrl_not_send_cb_called = 0;
+  /* Associated stream closed */
+  CU_ASSERT(0 == spdylay_session_send(session));
+  CU_ASSERT(1 == user_data.ctrl_not_send_cb_called);
+  CU_ASSERT(SPDYLAY_SYN_STREAM == user_data.not_sent_frame_type);
+  CU_ASSERT(SPDYLAY_ERR_STREAM_CLOSED == user_data.not_sent_error);
+
   /* Check SYN_REPLY */
   CU_ASSERT(0 ==
             spdylay_submit_syn_reply(session, SPDYLAY_CTRL_FLAG_FIN, 1, nv));
+  user_data.ctrl_not_send_cb_called = 0;
   CU_ASSERT(0 == spdylay_session_send(session));
   CU_ASSERT(1 == user_data.ctrl_not_send_cb_called);
   CU_ASSERT(SPDYLAY_SYN_REPLY == user_data.not_sent_frame_type);