diff --git a/ChangeLog b/ChangeLog
index 6cf3852e82..1914a90836 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,10 @@
+Tue Jan 31 21:27:43 2012  Narihiro Nakamura  <authornari@gmail.com>
+
+	* configure.in (HEAP_ALIGN_LOG): HEAP_ALIGN_LOG should be page
+	  size in OpenBSD. [ruby-core:42158][Bug #5901]
+
+	* gc.c : avoid to redefine.
+
 Tue Jan 31 14:27:22 2012  Nobuyoshi Nakada  <nobu@ruby-lang.org>
 
 	* test/ruby/envutil.rb (EnvUtil.invoke_ruby): yield also child pid
diff --git a/configure.in b/configure.in
index 085dfcfa20..2b56b749d4 100644
--- a/configure.in
+++ b/configure.in
@@ -1292,6 +1292,29 @@ main() {
 CFLAGS="$save_CFLAGS"])
 AC_DEFINE_UNQUOTED(GC_MARK_STACKFRAME_WORD, $rb_cv_gc_mark_stackframe_word)
 
+AS_CASE(["$target_os"],
+[openbsd*], [
+  AC_CACHE_CHECK(for heap align log on openbsd, rb_cv_page_size_log,
+    [rb_cv_page_size_log=no
+     for page_log in 12 13; do
+       AC_TRY_RUN([
+#include <math.h>
+#include <unistd.h>
+
+int
+main() {
+  if ((int)log2((double)sysconf(_SC_PAGESIZE)) != $page_log) return 1;
+  return 0;
+}
+       ],
+       rb_cv_page_size_log="$page_log"; break)
+     done])
+  if test $rb_cv_page_size_log != no; then
+    AC_DEFINE_UNQUOTED(HEAP_ALIGN_LOG, $rb_cv_page_size_log)
+  else
+    AC_DEFINE_UNQUOTED(HEAP_ALIGN_LOG, 12)
+  fi
+])
 
 dnl Checks for library functions.
 AC_TYPE_GETGROUPS
diff --git a/gc.c b/gc.c
index f52b5fc348..2c49f96363 100644
--- a/gc.c
+++ b/gc.c
@@ -536,8 +536,10 @@ rb_objspace_free(rb_objspace_t *objspace)
 }
 #endif
 
-/* tiny heap size: 16KB */
+#ifndef HEAP_ALIGN_LOG
+/* default tiny heap size: 16KB */
 #define HEAP_ALIGN_LOG 14
+#endif
 #define HEAP_ALIGN (1UL << HEAP_ALIGN_LOG)
 #define HEAP_ALIGN_MASK (~(~0UL << HEAP_ALIGN_LOG))
 #define REQUIRED_SIZE_BY_MALLOC (sizeof(size_t) * 5)