diff --git a/doc/extension.ja.rdoc b/doc/extension.ja.rdoc
index 88f300a562..9311bd70f8 100644
--- a/doc/extension.ja.rdoc
+++ b/doc/extension.ja.rdoc
@@ -1325,8 +1325,13 @@ void rb_define_hooked_variable(const char *name, VALUE *var, VALUE (*getter)(),
 
 void rb_global_variable(VALUE *var) ::
 
-  GCのため，Rubyプログラムからはアクセスされないが, Rubyオブ
-  ジェクトを含む大域変数をマークする．
+  マークする必要のあるRubyオブジェクトを含む大域変数を，GC
+  によって開放されないように保護する．
+
+void rb_gc_register_mark_object(VALUE *var) ::
+
+  マークする必要のあるRubyオブジェクトを，GCによって開放さ
+  れないように登録する．
 
 === 定数
 
diff --git a/doc/extension.rdoc b/doc/extension.rdoc
index 691631ea17..b7cba0d5e8 100644
--- a/doc/extension.rdoc
+++ b/doc/extension.rdoc
@@ -895,6 +895,10 @@ but are not exported to the Ruby world.  You need to protect them by
 
   void rb_global_variable(VALUE *var)
 
+or the objects themselves by
+
+  void rb_gc_register_mark_object(VALUE object)
+
 === Prepare extconf.rb
 
 If the file named extconf.rb exists, it will be executed to generate
@@ -1337,8 +1341,11 @@ void rb_define_hooked_variable(const char *name, VALUE *var, VALUE (*getter)(),
 
 void rb_global_variable(VALUE *var) ::
 
-  GC requires C global variables which hold Ruby values to be marked.
-  rb_global_variable tells GC to protect these variables.
+  Tells GC to protect C global variable, which holds Ruby value to be marked.
+
+void rb_gc_register_mark_object(VALUE object) ::
+
+  Tells GC to protect the +object+, which may not be referenced anywhere.
 
 === Constant Definition