diff --git a/lib/grn2drn/groonga-command-converter.rb b/lib/grn2drn/groonga-command-converter.rb
index 5f04680..bc414bf 100644
--- a/lib/grn2drn/groonga-command-converter.rb
+++ b/lib/grn2drn/groonga-command-converter.rb
@@ -17,6 +17,7 @@
 
 require "digest/sha1"
 require "time"
+require "json"
 
 require "groonga/command/parser"