diff --git a/src/shrpx_downstream.h b/src/shrpx_downstream.h
index 8536eb28..57f012f1 100644
--- a/src/shrpx_downstream.h
+++ b/src/shrpx_downstream.h
@@ -199,8 +199,6 @@ public:
 
   // Change the priority of downstream
   int change_priority(int32_t pri);
-
-  static const size_t OUTPUT_UPPER_THRES = 64*1024;
 private:
   Headers request_headers_;
   Headers response_headers_;
diff --git a/src/shrpx_http2_downstream_connection.cc b/src/shrpx_http2_downstream_connection.cc
index 857fa6be..2700938f 100644
--- a/src/shrpx_http2_downstream_connection.cc
+++ b/src/shrpx_http2_downstream_connection.cc
@@ -522,8 +522,8 @@ StreamData* Http2DownstreamConnection::detach_stream_data()
 bool Http2DownstreamConnection::get_output_buffer_full()
 {
   if(request_body_buf_) {
-    return
-      evbuffer_get_length(request_body_buf_) >= Downstream::OUTPUT_UPPER_THRES;
+    return http2session_->get_outbuf_length() +
+      evbuffer_get_length(request_body_buf_) >= Http2Session::OUTBUF_MAX_THRES;
   } else {
     return false;
   }
diff --git a/src/shrpx_http2_session.cc b/src/shrpx_http2_session.cc
index c7925a24..2b973ec3 100644
--- a/src/shrpx_http2_session.cc
+++ b/src/shrpx_http2_session.cc
@@ -697,7 +697,7 @@ ssize_t send_callback(nghttp2_session *session,
   auto bev = http2session->get_bev();
   auto output = bufferevent_get_output(bev);
   // Check buffer length and return WOULDBLOCK if it is large enough.
-  if(evbuffer_get_length(output) > Downstream::OUTPUT_UPPER_THRES) {
+  if(evbuffer_get_length(output) > Http2Session::OUTBUF_MAX_THRES) {
     return NGHTTP2_ERR_WOULDBLOCK;
   }
 
@@ -1369,4 +1369,13 @@ int Http2Session::terminate_session(nghttp2_error_code error_code)
   return 0;
 }
 
+size_t Http2Session::get_outbuf_length() const
+{
+  if(bev_) {
+    return evbuffer_get_length(bufferevent_get_output(bev_));
+  } else {
+    return OUTBUF_MAX_THRES;
+  }
+}
+
 } // namespace shrpx
diff --git a/src/shrpx_http2_session.h b/src/shrpx_http2_session.h
index 2abf7639..0d926112 100644
--- a/src/shrpx_http2_session.h
+++ b/src/shrpx_http2_session.h
@@ -104,6 +104,8 @@ public:
   int start_settings_timer();
   void stop_settings_timer();
 
+  size_t get_outbuf_length() const;
+
   enum {
     // Disconnected
     DISCONNECTED,
@@ -118,6 +120,8 @@ public:
     // Connected to downstream
     CONNECTED
   };
+
+  static const size_t OUTBUF_MAX_THRES = 64*1024;
 private:
   std::set<Http2DownstreamConnection*> dconns_;
   std::set<StreamData*> streams_;
diff --git a/src/shrpx_http_downstream_connection.cc b/src/shrpx_http_downstream_connection.cc
index 97fcfe48..8ce1ff62 100644
--- a/src/shrpx_http_downstream_connection.cc
+++ b/src/shrpx_http_downstream_connection.cc
@@ -37,6 +37,10 @@ using namespace nghttp2;
 
 namespace shrpx {
 
+namespace {
+const size_t OUTBUF_MAX_THRES = 64*1024;
+} // namespace
+
 // Workaround for the inability for Bufferevent to remove timeout from
 // bufferevent. Specify this long timeout instead of removing.
 namespace {
@@ -371,7 +375,7 @@ void HttpDownstreamConnection::force_resume_read()
 bool HttpDownstreamConnection::get_output_buffer_full()
 {
   auto output = bufferevent_get_output(bev_);
-  return evbuffer_get_length(output) >= Downstream::OUTPUT_UPPER_THRES;
+  return evbuffer_get_length(output) >= OUTBUF_MAX_THRES;
 }
 
 namespace {