diff --git a/ChangeLog b/ChangeLog
index c9b84963a9..388987102d 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,8 @@
+Tue Oct 30 03:08:53 2012  Nobuyoshi Nakada  <nobu@ruby-lang.org>
+
+	* lib/rbconfig/obsolete.rb (Config): re-introduce warnings for a
+	  lame-duck.  [ruby-core:46836] [Bug #6809]
+
 Tue Oct 30 02:20:10 2012  Aaron Patterson <aaron@tenderlovemaking.com>
 
 	* thread.c: added Thread#thread_variable_(get|set),
diff --git a/lib/rbconfig/obsolete.rb b/lib/rbconfig/obsolete.rb
new file mode 100644
index 0000000000..8669186083
--- /dev/null
+++ b/lib/rbconfig/obsolete.rb
@@ -0,0 +1,38 @@
+module ::RbConfig
+  module Obsolete
+  end
+  class << Obsolete
+    def _warn_
+      loc, = caller_locations(2, 1)
+      loc = "#{loc.to_s}: " if loc
+      warn "#{loc}Use RbConfig instead of obsolete and deprecated Config."
+      self
+    end
+
+    def const_missing(name)
+      _warn_
+      ::RbConfig.const_get(name)
+    end
+
+    def method_missing(*args, &block)
+      _warn_
+      rbconfig = ::RbConfig
+      result = rbconfig.__send__(*args, &block)
+      result = rbconfig if rbconfig.equal?(result)
+      result
+    end
+
+    def respond_to_missing?(*args, &block)
+      _warn_
+      ::RbConfig.respond_to_missing?(*args, &block)
+    end
+  end
+end
+
+::Config = ::RbConfig::Obsolete._warn_
+=begin
+def Object.const_missing(name)
+  return super unless name == :Config
+  ::RbConfig::Obsolete._warn_
+end
+=end
diff --git a/tool/mkconfig.rb b/tool/mkconfig.rb
index e5203c4d2a..f9d97d3d71 100755
--- a/tool/mkconfig.rb
+++ b/tool/mkconfig.rb
@@ -248,6 +248,7 @@ print <<EOS
     )
   end
 end
+autoload :Config, "rbconfig/obsolete.rb" # compatibility for ruby-1.8.4 and older.
 CROSS_COMPILING = nil unless defined? CROSS_COMPILING
 EOS