diff --git a/lib/droonga/command/serf_event_handler.rb b/lib/droonga/command/serf_event_handler.rb
index 6386cf01..8ba2f107 100644
--- a/lib/droonga/command/serf_event_handler.rb
+++ b/lib/droonga/command/serf_event_handler.rb
@@ -54,10 +54,10 @@ def parse_event
         @payload = nil
         case @event_name
         when "user"
-          @event_sub_name += ":#{ENV["SERF_USER_EVENT"]}"
+          @event_sub_name = ":#{ENV["SERF_USER_EVENT"]}"
           @payload = JSON.parse($stdin.gets)
         when "query"
-          @event_sub_name += ":#{ENV["SERF_USER_QUERY"]}"
+          @event_sub_name = ":#{ENV["SERF_USER_QUERY"]}"
           @payload = JSON.parse($stdin.gets)
         end
       end