diff --git a/ChangeLog b/ChangeLog
index 239d380afe..7821c559b9 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,8 @@
+Sun Nov  6 11:45:11 2016  Nobuyoshi Nakada  <nobu@ruby-lang.org>
+
+	* lib/e2mmap.rb (E2MM.def_exception): remove old definition if
+	  exists so that works multiple times.
+
 Sun Nov  6 10:03:00 2016  Shugo Maeda  <shugo@ruby-lang.org>
 
 	* test/ruby/test_optimization.rb (test_tailcall_interrupted_by_sigint):
diff --git a/lib/e2mmap.rb b/lib/e2mmap.rb
index e9cbd6a63d..ba1e87dd85 100644
--- a/lib/e2mmap.rb
+++ b/lib/e2mmap.rb
@@ -134,6 +134,7 @@ module Exception2MessageMapper
   def E2MM.def_exception(k, n, m, s = StandardError)
     e = Class.new(s)
     E2MM.instance_eval{@MessageMap[[k, e]] = m}
+    k.module_eval {remove_const(n)} if k.const_defined?(n, false)
     k.const_set(n, e)
   end