diff --git a/ChangeLog b/ChangeLog
index 8c2a38e716..ab260a6ef4 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,9 @@
+Wed Apr  3 20:52:49 2013  Tanaka Akira  <akr@fsij.org>
+
+	* process.c: Describe the behavior which Ruby invokes a commandline
+	  directly without shell if the commandline is simple enough.
+	  [ruby-core:50459] [Bug #7489]
+
 Wed Apr  3 20:27:37 2013  Tanaka Akira  <akr@fsij.org>
 
 	* ext/extmk.rb (extmake): Invoke Logging::log_close in a ensure
diff --git a/process.c b/process.c
index bb980701c1..5fd08c3e61 100644
--- a/process.c
+++ b/process.c
@@ -2349,6 +2349,12 @@ static int rb_exec_without_timer_thread(const struct rb_execarg *eargp, char *er
  *  The standard shell means always <code>"/bin/sh"</code> on Unix-like systems,
  *  <code>ENV["RUBYSHELL"]</code> or <code>ENV["COMSPEC"]</code> on Windows NT series, and
  *  similar.
+ *  If _commandline_ is simple enough,
+ *  no meta characters, no shell reserved word and no special built-in,
+ *  Ruby invokes the command directly without shell.
+ *  You can force shell invocation by adding ";" for _commandline_ (because ";" is a meta characetr).
+ *  Note that this behavior is observable by pid obtained
+ *  (return value of spawn() and IO#pid for IO.popen) is the pid of the invoked command, not shell.
  *
  *  If two or more +string+ given,
  *  the first is taken as a command name and