diff --git a/lib/fluent/plugin/base.rb b/lib/fluent/plugin/base.rb
index 3bd8f44224..fc2455f31a 100644
--- a/lib/fluent/plugin/base.rb
+++ b/lib/fluent/plugin/base.rb
@@ -52,14 +52,12 @@ def fluentd_worker_id
         @_fluentd_worker_id
       end
 
+      # @param conf [Fluent::Config::Element, Hash] configuration
       def configure(conf)
-        if Fluent::Engine.supervisor_mode || (conf.respond_to?(:for_this_worker?) && conf.for_this_worker?)
-          workers = if conf.target_worker_ids && !conf.target_worker_ids.empty?
-                      conf.target_worker_ids.size
-                    else
-                      1
-                    end
-          system_config_override(workers: workers)
+        if conf.is_a?(Fluent::Config::Element)
+          if (Fluent::Engine.supervisor_mode && !conf.target_worker_ids.empty?) || conf.for_this_worker?
+            system_config_override(workers: conf.target_worker_ids.size)
+          end
         end
         super(conf, system_config.strict_config_value)
         @_state ||= State.new(false, false, false, false, false, false, false, false, false)
diff --git a/test/plugin/test_base.rb b/test/plugin/test_base.rb
index b9567c9228..ffb5eb7667 100644
--- a/test/plugin/test_base.rb
+++ b/test/plugin/test_base.rb
@@ -146,4 +146,148 @@ class FluentPluginBaseTest::DummyPlugin2 < Fluent::Plugin::TestBase
       end
     end
   end
+
+  sub_test_case 'system_config.workers value after configure' do
+    sub_test_case 'with <system> workers 3 </system>' do
+      setup do
+        system_config = Fluent::SystemConfig.new
+        system_config.workers = 3
+        stub(Fluent::Engine).system_config { system_config }
+      end
+
+      sub_test_case 'supervisor_mode is true' do
+        setup do
+          stub(Fluent::Engine).supervisor_mode { true }
+          stub(Fluent::Engine).worker_id { -1 }
+        end
+
+        test 'without <worker> directive' do
+          conf = config_element()
+          conf.set_target_worker_ids([])
+          @p.configure(conf)
+          assert{ @p.system_config.workers == 3 }
+        end
+
+        test 'with <worker 0>' do
+          conf = config_element()
+          conf.set_target_worker_ids([0])
+          @p.configure(conf)
+          assert{ @p.system_config.workers == 1 }
+        end
+
+        test 'with <worker 0-1>' do
+          conf = config_element()
+          conf.set_target_worker_ids([0, 1])
+          @p.configure(conf)
+          assert{ @p.system_config.workers == 2 }
+        end
+
+        test 'with <worker 0-2>' do
+          conf = config_element()
+          conf.set_target_worker_ids([0, 1, 2])
+          @p.configure(conf)
+          assert{ @p.system_config.workers == 3 }
+        end
+
+        test '`conf` is `Hash`' do
+          @p.configure({})
+          assert{ @p.system_config.workers == 3 }
+        end
+      end
+
+      sub_test_case 'supervisor_mode is false' do
+        setup do
+          stub(Fluent::Engine).supervisor_mode { false }
+          stub(Fluent::Engine).worker_id { 0 }
+        end
+
+        test 'without <worker> directive' do
+          conf = config_element()
+          conf.set_target_worker_ids([])
+          @p.configure(conf)
+          assert{ @p.system_config.workers == 3 }
+        end
+
+        test 'with <worker 0>' do
+          conf = config_element()
+          conf.set_target_worker_ids([0])
+          @p.configure(conf)
+          assert{ @p.system_config.workers == 1 }
+        end
+
+        test 'with <worker 0-1>' do
+          conf = config_element()
+          conf.set_target_worker_ids([0, 1])
+          @p.configure(conf)
+          assert{ @p.system_config.workers == 2 }
+        end
+
+        test 'with <worker 0-2>' do
+          conf = config_element()
+          conf.set_target_worker_ids([0, 1, 2])
+          @p.configure(conf)
+          assert{ @p.system_config.workers == 3 }
+        end
+
+        test '`conf` is `Hash`' do
+          @p.configure({})
+          assert{ @p.system_config.workers == 3 }
+        end
+      end
+    end
+
+    sub_test_case 'without <system> directive' do
+      sub_test_case 'supervisor_mode is true' do
+        setup do
+          stub(Fluent::Engine).supervisor_mode { true }
+          stub(Fluent::Engine).worker_id { -1 }
+        end
+
+        test 'without <worker> directive' do
+          conf = config_element()
+          conf.set_target_worker_ids([])
+          @p.configure(conf)
+          assert{ @p.system_config.workers == 1 }
+        end
+
+        test 'with <worker 0>' do
+          conf = config_element()
+          conf.set_target_worker_ids([0])
+          @p.configure(conf)
+          assert{ @p.system_config.workers == 1 }
+        end
+
+        test '`conf` is `Hash`' do
+          @p.configure({})
+          assert{ @p.system_config.workers == 1 }
+        end
+      end
+
+      sub_test_case 'supervisor_mode is false' do
+        setup do
+          stub(Fluent::Engine).supervisor_mode { false }
+          stub(Fluent::Engine).worker_id { 0 }
+        end
+
+        test 'without <worker> directive' do
+          conf = config_element()
+          conf.set_target_worker_ids([])
+          @p.configure(conf)
+          assert{ @p.system_config.workers == 1 }
+        end
+
+        test 'with <worker 0>' do
+          conf = config_element()
+          conf.set_target_worker_ids([0])
+          @p.configure(conf)
+          assert{ @p.system_config.workers == 1 }
+        end
+
+        test '`conf` is `Hash`' do
+          @p.configure({})
+          assert{ @p.system_config.workers == 1 }
+        end
+      end
+    end
+  end
 end