diff --git a/ChangeLog b/ChangeLog
index 8fd3ed7e44..820a3e32d6 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,7 @@
+Sun May 12 15:50:45 2013  KOSAKI Motohiro  <kosaki.motohiro@gmail.com>
+
+	* configure.in: adds comments for setjmp check.
+
 Sun May 12 15:38:09 2013  KOSAKI Motohiro  <kosaki.motohiro@gmail.com>
 
 	* configure.in: move clock_gettime() check into regular place.
diff --git a/configure.in b/configure.in
index 8cb98baf35..5f392b253e 100644
--- a/configure.in
+++ b/configure.in
@@ -1724,8 +1724,8 @@ else
 fi
 
 AC_CHECK_FUNCS(__syscall)
-AC_CHECK_FUNCS(_longjmp)
-AC_CHECK_FUNCS(_setjmp)
+AC_CHECK_FUNCS(_longjmp)		# used for AC_ARG_WITH(setjmp-type)
+AC_CHECK_FUNCS(_setjmp)			# used for AC_ARG_WITH(setjmp-type)
 AC_CHECK_FUNCS(chroot)
 AC_CHECK_FUNCS(chsize)
 AC_CHECK_FUNCS(clock_gettime)
@@ -1836,13 +1836,6 @@ if test x"$ac_cv_func_clock_gettime" != xyes; then
     AC_CHECK_FUNCS(clock_gettime)
 fi
 
-AC_CACHE_CHECK(for sigsetjmp as a macro or function, ac_cv_func_sigsetjmp,
-  [AC_TRY_COMPILE([
-#include <setjmp.h>
-], [sigjmp_buf env; sigsetjmp(env,1);],
-	ac_cv_func_sigsetjmp=yes,
-	ac_cv_func_sigsetjmp=no)])
-
 AC_CACHE_CHECK(for unsetenv returns a value, rb_cv_unsetenv_return_value,
   [AC_TRY_COMPILE([
 #include <stdlib.h>
@@ -1853,6 +1846,14 @@ if test "$rb_cv_unsetenv_return_value" = no; then
   AC_DEFINE(VOID_UNSETENV)
 fi
 
+# used for AC_ARG_WITH(setjmp-type)
+AC_CACHE_CHECK(for sigsetjmp as a macro or function, ac_cv_func_sigsetjmp,
+  [AC_TRY_COMPILE([
+#include <setjmp.h>
+], [sigjmp_buf env; sigsetjmp(env,1);],
+	ac_cv_func_sigsetjmp=yes,
+	ac_cv_func_sigsetjmp=no)])
+
 AC_CACHE_CHECK(for __builtin_setjmp, ac_cv_func___builtin_setjmp,
 [AC_TRY_LINK([@%:@include <setjmp.h>
     jmp_buf jb; void t(v) int v; {__builtin_longjmp(jb, v);}],
@@ -1861,6 +1862,7 @@ AC_CACHE_CHECK(for __builtin_setjmp, ac_cv_func___builtin_setjmp,
     [ac_cv_func___builtin_setjmp=no])
 ])
 
+# we don't use _setjmp if _longjmp doesn't exist.
 test x$ac_cv_func__longjmp = xno && ac_cv_func__setjmp=no
 
 AC_MSG_CHECKING(for setjmp type)
@@ -1896,6 +1898,7 @@ AC_MSG_RESULT(${setjmp_prefix}setjmp)
 AC_DEFINE_UNQUOTED([RUBY_SETJMP(env)], [${setjmp_prefix}setjmp(env${setjmp_sigmask+,0})])
 AC_DEFINE_UNQUOTED([RUBY_LONGJMP(env,val)], [${setjmp_prefix}longjmp(env,val)])
 AC_DEFINE_UNQUOTED(RUBY_JMP_BUF, ${setjmp_sigmask+${setjmp_prefix}}jmp_buf)
+# End of setjmp check.
 
 AC_ARG_ENABLE(setreuid,
        AS_HELP_STRING([--enable-setreuid], [use setreuid()/setregid() according to need even if obsolete]),