diff --git a/app/form_builders/fluentd_form_builder.rb b/app/form_builders/fluentd_form_builder.rb
index 3db57d8..0bcb91a 100644
--- a/app/form_builders/fluentd_form_builder.rb
+++ b/app/form_builders/fluentd_form_builder.rb
@@ -34,7 +34,7 @@ class FluentdFormBuilder < ActionView::Helpers::FormBuilder
 
   def enum_field(key, options)
     label(key, nil, data: { toggle: "tooltip", placement: "right" }, title: object.desc(key)) +
-      select(key, object.list_of(key), options, { class: "enum" })
+      select(key, object.list_of(key), options, { class: "enum form-control" })
   end
 
   def bool_field(key, options)