diff --git a/lib/includes/nghttp2/nghttp2.h b/lib/includes/nghttp2/nghttp2.h
index e4b865e6..ba4729ef 100644
--- a/lib/includes/nghttp2/nghttp2.h
+++ b/lib/includes/nghttp2/nghttp2.h
@@ -123,7 +123,14 @@ typedef struct {
  *
  * The maximum weight of priority group.
  */
-#define NGHTTP2_MAX_WEIGHT 255
+#define NGHTTP2_MAX_WEIGHT 256
+
+/**
+ * @macro
+ *
+ * The minimum weight of priority group.
+ */
+#define NGHTTP2_MIN_WEIGHT 1
 
 /**
  * @macro
@@ -717,7 +724,7 @@ typedef struct {
   /**
    * The weight of the priority group
    */
-  uint8_t weight;
+  int32_t weight;
 } nghttp2_priority_group;
 
 /**
@@ -2073,13 +2080,21 @@ const char* nghttp2_strerror(int lib_error_code);
  * @function
  *
  * Initializes |pri_spec| with priority group ID |pri_group_id| and
- * its weight |weight|.  To specify weight for the default priority
- * group (which is the same as the stream ID of the stream) in
- * `nghttp2_submit_request()` and `nghttp2_submit_headers()` and its
- * stream ID is not known in advance, specify -1 to |pri_group_id|.
+ * its weight |weight|.
+ *
+ * The |weight| must be in [:enum:`NGHTTP2_MIN_WEIGHT`,
+ * :enum:`NGHTTP2_MAX_WEIGHT`], inclusive.  If |weight| is strictly
+ * less than :enum:`NGHTTP2_MIN_WEIGHT`, it becomes
+ * :enum:`NGHTTP2_MIN_WEIGHT`.  If it is strictly greater than
+ * :enum:`NGHTTP2_MAX_WEIGHT`, it becomes :enum:`NGHTTP2_MAX_WEIGHT`.
+ *
+ * To specify weight for the default priority group (which is the same
+ * as the stream ID of the stream) in `nghttp2_submit_request()` and
+ * `nghttp2_submit_headers()` and its stream ID is not known in
+ * advance, specify -1 to |pri_group_id|.
  */
 void nghttp2_priority_spec_group_init(nghttp2_priority_spec *pri_spec,
-                                      int32_t pri_group_id, uint8_t weight);
+                                      int32_t pri_group_id, int32_t weight);
 
 /**
  * @function
diff --git a/lib/nghttp2_frame.c b/lib/nghttp2_frame.c
index 1ef92de0..621d63f6 100644
--- a/lib/nghttp2_frame.c
+++ b/lib/nghttp2_frame.c
@@ -390,7 +390,7 @@ void nghttp2_frame_pack_priority_spec(uint8_t *buf,
   case NGHTTP2_PRIORITY_TYPE_GROUP:
 
     nghttp2_put_uint32be(buf, pri_spec->group.pri_group_id);
-    buf[4] = pri_spec->group.weight;
+    buf[4] = pri_spec->group.weight - 1;
 
     return;
   case NGHTTP2_PRIORITY_TYPE_DEP:
@@ -413,10 +413,10 @@ void nghttp2_frame_unpack_priority_spec(nghttp2_priority_spec *pri_spec,
 {
   if(flags & NGHTTP2_FLAG_PRIORITY_GROUP) {
     int32_t pri_group_id;
-    uint8_t weight;
+    int32_t weight;
 
     pri_group_id = nghttp2_get_uint32(payload) & NGHTTP2_PRI_GROUP_ID_MASK;
-    weight = payload[4];
+    weight = payload[4] + 1;
 
     nghttp2_priority_spec_group_init(pri_spec, pri_group_id, weight);
   } else if(flags & NGHTTP2_FLAG_PRIORITY_DEPENDENCY) {
diff --git a/lib/nghttp2_outbound_item.h b/lib/nghttp2_outbound_item.h
index 37245f64..50cbb4f3 100644
--- a/lib/nghttp2_outbound_item.h
+++ b/lib/nghttp2_outbound_item.h
@@ -33,11 +33,11 @@
 #include "nghttp2_frame.h"
 
 /* A bit higher weight for non-DATA frames */
-#define NGHTTP2_OB_EX_WEIGHT 256
+#define NGHTTP2_OB_EX_WEIGHT 300
 /* Higher weight for SETTINGS */
-#define NGHTTP2_OB_SETTINGS_WEIGHT 257
+#define NGHTTP2_OB_SETTINGS_WEIGHT 301
 /* Highest weight for PING */
-#define NGHTTP2_OB_PING_WEIGHT 258
+#define NGHTTP2_OB_PING_WEIGHT 302
 
 typedef struct {
   nghttp2_data_provider *data_prd;
diff --git a/lib/nghttp2_priority_spec.c b/lib/nghttp2_priority_spec.c
index 9c9028e7..8fcfba85 100644
--- a/lib/nghttp2_priority_spec.c
+++ b/lib/nghttp2_priority_spec.c
@@ -25,7 +25,7 @@
 #include "nghttp2_priority_spec.h"
 
 void nghttp2_priority_spec_group_init(nghttp2_priority_spec *pri_spec,
-                                      int32_t pri_group_id, uint8_t weight)
+                                      int32_t pri_group_id, int32_t weight)
 {
   pri_spec->pri_type = NGHTTP2_PRIORITY_TYPE_GROUP;
   pri_spec->group.pri_group_id = pri_group_id;
diff --git a/lib/nghttp2_session.c b/lib/nghttp2_session.c
index 911c520a..2bf1e4d8 100644
--- a/lib/nghttp2_session.c
+++ b/lib/nghttp2_session.c
@@ -748,7 +748,7 @@ nghttp2_stream* nghttp2_session_open_stream(nghttp2_session *session,
   nghttp2_stream *dep_stream;
   nghttp2_stream *root_stream;
   int32_t pri_group_id;
-  uint8_t weight;
+  int32_t weight;
   nghttp2_stream_group *stream_group;
 
   dep_stream = NULL;
@@ -1883,7 +1883,7 @@ static int session_call_on_frame_send(nghttp2_session *session,
 static void outbound_item_cycle_weight(nghttp2_outbound_item *item,
                                        int32_t ini_weight)
 {
-  if(item->weight == 0 || item->weight > ini_weight) {
+  if(item->weight == NGHTTP2_MIN_WEIGHT || item->weight > ini_weight) {
     item->weight = ini_weight;
   } else {
     --item->weight;
diff --git a/lib/nghttp2_submit.c b/lib/nghttp2_submit.c
index 117fda44..fd77c907 100644
--- a/lib/nghttp2_submit.c
+++ b/lib/nghttp2_submit.c
@@ -102,6 +102,17 @@ static int nghttp2_submit_headers_shared
   return rv;
 }
 
+static void adjust_priority_spec_group_weight(nghttp2_priority_spec *pri_spec)
+{
+  assert(pri_spec->pri_type == NGHTTP2_PRIORITY_TYPE_GROUP);
+
+  if(pri_spec->group.weight < NGHTTP2_MIN_WEIGHT) {
+    pri_spec->group.weight = NGHTTP2_MIN_WEIGHT;
+  } else if(pri_spec->group.weight > NGHTTP2_MAX_WEIGHT) {
+    pri_spec->group.weight = NGHTTP2_MAX_WEIGHT;
+  }
+}
+
 static int nghttp2_submit_headers_shared_nva
 (nghttp2_session *session,
  uint8_t flags,
@@ -114,24 +125,25 @@ static int nghttp2_submit_headers_shared_nva
 {
   ssize_t rv;
   nghttp2_nv *nva_copy;
-  nghttp2_priority_spec pri_spec_none = {
+  nghttp2_priority_spec copy_pri_spec = {
     NGHTTP2_PRIORITY_TYPE_NONE
   };
-  const nghttp2_priority_spec *pri_spec_ptr;
 
   rv = nghttp2_nv_array_copy(&nva_copy, nva, nvlen);
   if(rv < 0) {
     return rv;
   }
 
-  if(pri_spec == NULL) {
-    pri_spec_ptr = &pri_spec_none;
-  } else {
-    pri_spec_ptr = pri_spec;
+  if(pri_spec) {
+    copy_pri_spec = *pri_spec;
+
+    if(copy_pri_spec.pri_type == NGHTTP2_PRIORITY_TYPE_GROUP) {
+      adjust_priority_spec_group_weight(&copy_pri_spec);
+    }
   }
 
   return nghttp2_submit_headers_shared(session, flags, stream_id,
-                                       pri_spec_ptr, nva_copy, rv, data_prd,
+                                       &copy_pri_spec, nva_copy, rv, data_prd,
                                        stream_user_data);
 }
 
@@ -176,16 +188,21 @@ int nghttp2_submit_priority(nghttp2_session *session, uint8_t flags,
 {
   int rv;
   nghttp2_frame *frame;
+  nghttp2_priority_spec copy_pri_spec;
 
   if(pri_spec == NULL) {
     return NGHTTP2_ERR_INVALID_ARGUMENT;
   }
 
-  switch(pri_spec->pri_type) {
+  copy_pri_spec = *pri_spec;
+
+  switch(copy_pri_spec.pri_type) {
   case NGHTTP2_PRIORITY_TYPE_GROUP:
+    adjust_priority_spec_group_weight(&copy_pri_spec);
+
     break;
   case NGHTTP2_PRIORITY_TYPE_DEP:
-    if(stream_id == pri_spec->dep.stream_id) {
+    if(stream_id == copy_pri_spec.dep.stream_id) {
       return NGHTTP2_ERR_INVALID_ARGUMENT;
     }
 
@@ -200,7 +217,7 @@ int nghttp2_submit_priority(nghttp2_session *session, uint8_t flags,
     return NGHTTP2_ERR_NOMEM;
   }
 
-  nghttp2_frame_priority_init(&frame->priority, stream_id, pri_spec);
+  nghttp2_frame_priority_init(&frame->priority, stream_id, &copy_pri_spec);
 
   rv = nghttp2_session_add_frame(session, NGHTTP2_CAT_CTRL, frame, NULL);