diff --git a/grntest.gemspec b/grntest.gemspec
index 2b6ae3b..fa5bcda 100644
--- a/grntest.gemspec
+++ b/grntest.gemspec
@@ -49,7 +49,7 @@ Gem::Specification.new do |spec|
   spec.add_runtime_dependency("groonga-log")
   spec.add_runtime_dependency("groonga-query-log", ">= 1.4.1")
   spec.add_runtime_dependency("groonga-synonym")
-  spec.add_runtime_dependency("json")
+  spec.add_runtime_dependency("json", ">= 2.8.1")
   spec.add_runtime_dependency("msgpack")
   spec.add_runtime_dependency("rexml")