diff --git a/ChangeLog b/ChangeLog
index 44f6fcef5b..33b9f978cb 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,7 @@
+Mon Jan 21 15:41:33 2013  Nobuyoshi Nakada  <nobu@ruby-lang.org>
+
+	* tool/mkconfig.rb: BASERUBY is transient at core build.
+
 Mon Jan 21 13:51:21 2013  Nobuyoshi Nakada  <nobu@ruby-lang.org>
 
 	* lib/mkmf.rb ($extmk): traverse parent directories for the case
diff --git a/common.mk b/common.mk
index 1abef18031..531ba26daa 100644
--- a/common.mk
+++ b/common.mk
@@ -936,7 +936,7 @@ $(srcdir)/ext/ripper/ripper.c: parse.y
 $(srcdir)/ext/json/parser/parser.c: $(srcdir)/ext/json/parser/parser.rl
 	$(ECHO) generating $@
 	$(Q) $(CHDIR) $(@D) && $(exec) $(MAKE) -f prereq.mk $(MFLAGS) \
-		Q=$(Q) ECHO=$(ECHO) top_srcdir=../../.. srcdir=.
+		Q=$(Q) ECHO=$(ECHO) top_srcdir=../../.. srcdir=. VPATH=../../.. BASERUBY="$(BASERUBY)"
 
 $(srcdir)/ext/dl/callback/callback.c: $(srcdir)/ext/dl/callback/mkcallback.rb $(srcdir)/ext/dl/dl.h
 	$(ECHO) generating $@
diff --git a/tool/mkconfig.rb b/tool/mkconfig.rb
index e73cd29d62..faa1a3f458 100755
--- a/tool/mkconfig.rb
+++ b/tool/mkconfig.rb
@@ -73,7 +73,7 @@ File.foreach "config.status" do |line|
   if name
     case name
     when /^(?:ac_.*|configure_input|(?:top_)?srcdir|\w+OBJS)$/; next
-    when /^(?:X|(?:MINI|RUN)RUBY$)/; next
+    when /^(?:X|(?:MINI|RUN|BASE)RUBY$)/; next
     when /^(?:MAJOR|MINOR|TEENY)$/; next
     when /^RUBY_INSTALL_NAME$/; next if $install_name
     when /^RUBY_SO_NAME$/; next if $so_name
@@ -132,7 +132,7 @@ File.foreach "config.status" do |line|
         val.sub!(/universal/, %q[#{arch && universal[/(?:\A|\s)#{Regexp.quote(arch)}=(\S+)/, 1] || '\&'}])
       end
     end
-    v = "  CONFIG[\"#{name}\"] #{win32 && vars[name] ? '<< "\n" +' : '='} #{val}\n"
+    v = "  CONFIG[\"#{name}\"] #{win32 && vars[name] ? '<< "\n"' : '='} #{val}\n"
     vars[name] = true
     if fast[name]
       v_fast << v