diff --git a/travis_config.rb b/travis_config.rb
index 6f406ae378..ffa0a75d4a 100644
--- a/travis_config.rb
+++ b/travis_config.rb
@@ -4,6 +4,7 @@
 
   # include all core GEMs
   conf.gembox 'full-core'
+  conf.cc.flags += %w(-Werror=declaration-after-statement)
   conf.cc.defines += %w(MRB_GC_FIXED_ARENA)
 end
 
@@ -12,6 +13,7 @@
 
   # include all core GEMs
   conf.gembox 'full-core'
+  conf.cc.flags += %w(-Werror=declaration-after-statement)
   conf.cc.defines = %w(MRB_DEBUG MRB_GC_FIXED_ARENA)
   conf.enable_bintest = true
 end