diff --git a/ChangeLog b/ChangeLog
index 18b5819752..cf457d3830 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,9 @@
+Thu Dec 27 22:31:37 2007  NAKAMURA Usaku  <usa@ruby-lang.org>
+
+	* lib/rubygems/commands/update_command.rb (do_rubygems_update): use
+	  portable and safely ENV operation. reported in
+	  <http://slashdot.jp/developers/comments.pl?sid=384937&cid=1273085>.
+
 Thu Dec 27 21:47:04 2007  NAKAMURA Usaku  <usa@ruby-lang.org>
 
 	* mkconfig.rb (prefix): archdir is "1.9.0", not "1.9". reported in
diff --git a/lib/rubygems/commands/update_command.rb b/lib/rubygems/commands/update_command.rb
index 7a11ec9554..88d48d705e 100644
--- a/lib/rubygems/commands/update_command.rb
+++ b/lib/rubygems/commands/update_command.rb
@@ -122,11 +122,10 @@ class Gem::Commands::UpdateCommand < Gem::Command
       setup_cmd = "#{Gem.ruby} setup.rb #{args.join ' '}"
 
       # Make sure old rubygems isn't loaded
-      if Gem.win_platform? then
-        system "set RUBYOPT= & #{setup_cmd}"
-      else
-        system "RUBYOPT=\"\" #{setup_cmd}"
-      end
+      old = ENV["RUBYOPT"]
+      ENV.delete("RUBYOPT")
+      system setup_cmd
+      ENV["RUBYOPT"] = old if old
     end
   end