diff --git a/.rubocop.yml b/.rubocop.yml
index f54f2a7..5385854 100644
--- a/.rubocop.yml
+++ b/.rubocop.yml
@@ -5,7 +5,7 @@ require:
 
 AllCops:
   TargetRubyVersion: 2.7
-  TargetRailsVersion: 5.2
+  TargetRailsVersion: 6.1
   NewCops: enable
 
 Rails:
diff --git a/lib/tasks/redmine_plugin_kit.rake b/lib/tasks/redmine_plugin_kit.rake
index d10e251..76c1459 100644
--- a/lib/tasks/redmine_plugin_kit.rake
+++ b/lib/tasks/redmine_plugin_kit.rake
@@ -45,7 +45,7 @@ namespace :redmine_plugin_kit do
     if name == 'redmine'
       Setting[setting.to_sym] = value
     else
-      plugin_name = "plugin_#{name}".to_sym
+      plugin_name = :"plugin_#{name}"
       plugin_settings = Setting[plugin_name]
       plugin_settings[setting] = value
       Setting[plugin_name] = plugin_settings
@@ -74,7 +74,7 @@ namespace :redmine_plugin_kit do
     if name == 'redmine'
       puts Setting.send(setting)
     else
-      plugin_name = "plugin_#{name}".to_sym
+      plugin_name = :"plugin_#{name}"
       plugin_settings = Setting[plugin_name]
       puts plugin_settings[setting]
     end
diff --git a/test/units/loader_test.rb b/test/units/loader_test.rb
index a9cddc8..c641c4d 100644
--- a/test/units/loader_test.rb
+++ b/test/units/loader_test.rb
@@ -144,13 +144,13 @@ def test_do_not_allow_helper_if_controller_patch_exists_as_hash
   def test_load_model_hooks
     hooks = RedminePluginKit::Loader.new(plugin_id: @plugin_id).load_model_hooks!
 
-    assert hooks.is_a? Module
+    assert_kind_of Module, hooks
   end
 
   def test_load_hooks
     hooks = RedminePluginKit::Loader.new(plugin_id: @plugin_id).load_view_hooks!
 
-    assert hooks.is_a? Module
+    assert_kind_of Module, hooks
   end
 
   def test_load_macros