diff --git a/configure.ac b/configure.ac
index 062d5cb9..6bc25f4d 100644
--- a/configure.ac
+++ b/configure.ac
@@ -25,7 +25,7 @@ dnl Do not change user variables!
 dnl http://www.gnu.org/software/automake/manual/html_node/Flag-Variables-Ordering.html
 
 AC_PREREQ(2.61)
-AC_INIT([nghttp2], [1.5.0], [t-tujikawa@users.sourceforge.net])
+AC_INIT([nghttp2], [1.5.1-DEV], [t-tujikawa@users.sourceforge.net])
 AC_CONFIG_AUX_DIR([.])
 AC_CONFIG_MACRO_DIR([m4])
 AC_CONFIG_HEADERS([config.h])