diff --git a/ChangeLog b/ChangeLog
index 6d1dfbf3a0..f9407e99da 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,7 @@
+Tue Sep 23 17:50:35 2008  Koichi Sasada  <ko1@atdot.net>
+
+	* thread.c (rb_thread_blocking_region): write a document
+
 Tue Sep 23 17:14:31 2008  Koichi Sasada  <ko1@atdot.net>
 
 	* thread.c, include/ruby/intern.h (rb_thread_interrupted): added.
diff --git a/thread.c b/thread.c
index 7416fabb25..8eb0597b83 100644
--- a/thread.c
+++ b/thread.c
@@ -939,6 +939,34 @@ rb_thread_schedule(void)
     }
 }
 
+/*
+ * rb_thread_blocking_region - permit concurrent/parallel execution.
+ *
+ * This function does:
+ *   (1) release GVL.
+ *       Other Ruby threads may run in parallel.
+ *   (2) call func with data1.
+ *   (3) aquire GVL.
+ *       Other Ruby threads can not run in parallel any more.
+ *
+ *   If another thread interrupts this thread (Thread#kill, signal deliverly,
+ *   VM-shutdown request, and so on), `ubf()' is called (`ubf()' means
+ *   "un-blocking function").  `ubf()' should interrupt `func()' execution.
+ *   There are built-in ubfs and you can specify these ubfs.
+ *   However, we can not guarantee our built-in ubfs interrupt
+ *   your `func()' correctly.  Becareful to use rb_thread_blocking_region().
+ *
+ *     * RUBY_UBF_IO: ubf for IO operation
+ *     * RUBY_UBF_PROCESS: ubf for process operation
+ *
+ *   NOTE: You can not execute most of Ruby C API and touch Ruby objects
+ *         in `func()' and `ubf()' because current thread doesn't acquire
+ *         GVL (cause synchronization problem).  If you need to do it,
+ *         read source code of C APIs and confirm by yourself.
+ *
+ *   Safe C API:
+ *     * rb_thread_interrupted() - check interrupt flag
+ */
 VALUE
 rb_thread_blocking_region(
     rb_blocking_function_t *func, void *data1,