diff --git a/test/plugin/test_in_object_space.rb b/test/plugin/test_in_object_space.rb
index 6e8bfa524b..64e3c74e06 100644
--- a/test/plugin/test_in_object_space.rb
+++ b/test/plugin/test_in_object_space.rb
@@ -50,6 +50,10 @@ def test_configure
   end
 
   def test_emit
+    # Force release garbaged objects due to avoid unexpected error by mock objects on `on_timer`
+    # https://github.com/fluent/fluentd/pull/5055
+    GC.start
+
     d = create_driver
 
     d.run(expect_emits: 3)
diff --git a/test/plugin/test_input.rb b/test/plugin/test_input.rb
index d6e68599d9..98e5b5e2e5 100644
--- a/test/plugin/test_input.rb
+++ b/test/plugin/test_input.rb
@@ -133,5 +133,7 @@ def emit(tag, es)
     assert{ @p.router.object_id != original_router.object_id }
 
     @p.router.emit('mytag.testing', ['for mock'])
+  ensure
+    Fluent::Engine.root_agent.labels['@mytest'] = nil
   end
 end
diff --git a/test/plugin/test_out_forward.rb b/test/plugin/test_out_forward.rb
index c2442b631a..03cea05c37 100644
--- a/test/plugin/test_out_forward.rb
+++ b/test/plugin/test_out_forward.rb
@@ -344,7 +344,7 @@ def try_write(chunk)
   end
 
   test 'set_compress_is_gzip_in_buffer_section' do
-    mock = flexmock($log)
+    mock = flexmock($log.dup)
     mock.should_receive(:log).with("buffer is compressed.  If you also want to save the bandwidth of a network, Add `compress` configuration in <match>")
 
     @d = d = create_driver(config + %[
diff --git a/test/plugin_helper/test_event_emitter.rb b/test/plugin_helper/test_event_emitter.rb
index 409b9800bf..4e7543210a 100644
--- a/test/plugin_helper/test_event_emitter.rb
+++ b/test/plugin_helper/test_event_emitter.rb
@@ -70,6 +70,8 @@ class Dummy < Fluent::Plugin::TestBase
     d.configure(config_element('ROOT', '', {'@label' => '@mytest'}))
     router = d.event_emitter_router("@mytest")
     assert_equal router_mock, router
+  ensure
+    Fluent::Engine.root_agent.labels['@mytest'] = nil
   end
 
   test 'get root router' do