diff --git a/ChangeLog b/ChangeLog
index 80ffcb5f4e..083d8ca062 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,7 @@
+Sat Jun 26 13:07:09 2010  Nobuyoshi Nakada  <nobu@ruby-lang.org>
+
+	* configure.in (enable-debug-env): option for RUBY_DEBUG env.
+
 Sat Jun 26 11:56:31 2010  Nobuyoshi Nakada  <nobu@ruby-lang.org>
 
 	* common.mk (clear-installed-list): put redirection before set
diff --git a/configure.in b/configure.in
index a16a3d65b3..4098b34c3d 100644
--- a/configure.in
+++ b/configure.in
@@ -2277,6 +2277,10 @@ AS_CASE(["$THREAD_MODEL"],
 [""],      [AC_MSG_ERROR(thread model is missing)],
            [AC_MSG_ERROR(unknown thread model $THREAD_MODEL)])
 
+AC_ARG_ENABLE(debug-env,
+       AS_HELP_STRING([--enable-debug-env], [enable RUBY_DEBUG environment variable]),
+       [AC_DEFINE(RUBY_DEBUG_ENV)])
+
 AS_CASE(["$FIRSTMAKEFILE"], [*GNUmakefile:*], [gnumake=yes], [
     AC_MSG_CHECKING([for if ${MAKE-make} is GNU make])
     mkdir conftest.dir
diff --git a/win32/Makefile.sub b/win32/Makefile.sub
index 8cbe212e4a..acfcd1761e 100644
--- a/win32/Makefile.sub
+++ b/win32/Makefile.sub
@@ -584,6 +584,9 @@ $(CONFIG_H): $(MKFILES) $(srcdir)/win32/Makefile.sub $(win_srcdir)/Makefile.sub
 !endif
 #define CANONICALIZATION_FOR_MATHN 1
 #define DEFAULT_KCODE KCODE_NONE
+!if "$(ENABLE_DEBUG_ENV)" == "yes"
+#define RUBY_DEBUG_ENV 1
+!endif
 #define LOAD_RELATIVE 1
 #define DLEXT ".so"
 #define EXECUTABLE_EXTS $(EXECUTABLE_EXTS)
diff --git a/win32/configure.bat b/win32/configure.bat
index 0401b61dfd..02e85640f0 100755
--- a/win32/configure.bat
+++ b/win32/configure.bat
@@ -27,6 +27,8 @@ if "%1" == "--enable-install-doc" goto :enable-rdoc
 if "%1" == "--disable-install-doc" goto :disable-rdoc
 if "%1" == "--enable-win95" goto :enable-win95
 if "%1" == "--disable-win95" goto :disable-win95
+if "%1" == "--enable-debug-env" goto :enable-debug-env
+if "%1" == "--disable-debug-env" goto :disable-debug-env
 if "%1" == "--extout" goto :extout
 if "%1" == "--path" goto :path
 if "%1" == "--with-baseruby" goto :baseruby
@@ -114,6 +116,16 @@ goto :loop
   echo>>confargs.tmp %1 \
   shift
 goto :loop
+:enable-debug-env
+  echo>> ~tmp~.mak 	"ENABLE_DEBUG_ENV=yes" \
+  echo>>confargs.tmp %1 \
+  shift
+goto :loop
+:disable-debug-env
+  echo>> ~tmp~.mak 	"ENABLE_DEBUG_ENV=no" \
+  echo>>confargs.tmp %1 \
+  shift
+goto :loop
 :ntver
   echo>> ~tmp~.mak 	"NTVER=%2" \
   echo>>confargs.tmp %1=%2 \