diff --git a/lib/fluent/error.rb b/lib/fluent/error.rb
index 564b1c8d63..251094c820 100644
--- a/lib/fluent/error.rb
+++ b/lib/fluent/error.rb
@@ -28,6 +28,9 @@ def to_s
   class InvalidRootDirectory < UnrecoverableError
   end
 
+  class InvalidLockDirectory < UnrecoverableError
+  end
+
   # For internal use
   class UncatchableError < Exception
   end
diff --git a/lib/fluent/plugin/base.rb b/lib/fluent/plugin/base.rb
index 601e216b81..1846273a17 100644
--- a/lib/fluent/plugin/base.rb
+++ b/lib/fluent/plugin/base.rb
@@ -31,6 +31,7 @@ class Base
       def initialize
         @log = nil
         super
+        @fluentd_lock_dir = ENV['FLUENTD_LOCK_DIR']
         @_state = State.new(false, false, false, false, false, false, false, false, false)
         @_context_router = nil
         @_fluentd_worker_id = nil
@@ -70,6 +71,21 @@ def multi_workers_ready?
         true
       end
 
+      def get_lock_path(name)
+        name = name.gsub(/[^a-zA-Z0-9]/, "_")
+        File.join(@fluentd_lock_dir, "fluentd-#{name}.lock")
+      end
+
+      def acquire_worker_lock(name)
+        if @fluentd_lock_dir.nil?
+          raise InvalidLockDirectory, "can't acquire lock because FLUENTD_LOCKDIR isn't set"
+        end
+        File.open(get_lock_path(name), "w") do |f|
+          f.flock(File::LOCK_EX)
+          yield
+        end
+      end
+
       def string_safe_encoding(str)
         unless str.valid_encoding?
           str = str.scrub('?')
diff --git a/lib/fluent/plugin/out_file.rb b/lib/fluent/plugin/out_file.rb
index f09c149eb1..7bf25a1b6e 100644
--- a/lib/fluent/plugin/out_file.rb
+++ b/lib/fluent/plugin/out_file.rb
@@ -188,6 +188,10 @@ def configure(conf)
         condition = Gem::Dependency.new('', [">= 2.7.0", "< 3.1.0"])
         @need_ruby_on_macos_workaround = true if condition.match?('', RUBY_VERSION)
       end
+
+      if @need_lock && @append && fluentd_lockdir.nil?
+        raise InvalidLockDirectory, "must set FLUENTD_LOCKDIR on multi-worker append mode"
+      end
     end
 
     def multi_workers_ready?
@@ -217,7 +221,13 @@ def write(chunk)
                end
 
       if @append
-        writer.call(path, chunk)
+        if @need_lock
+          acquire_worker_lock(path) do
+            writer.call(path, chunk)
+          end
+        else
+          writer.call(path, chunk)
+        end
       else
         find_filepath_available(path, with_lock: @need_lock) do |actual_path|
           writer.call(actual_path, chunk)
diff --git a/lib/fluent/supervisor.rb b/lib/fluent/supervisor.rb
index 0bd82ece47..854e53cb12 100644
--- a/lib/fluent/supervisor.rb
+++ b/lib/fluent/supervisor.rb
@@ -875,7 +875,11 @@ def supervise
       se = ServerEngine.create(ServerModule, WorkerModule){
         Fluent::Supervisor.load_config(@config_path, params)
       }
-      se.run
+
+      Dir.mktmpdir("fluentd-lock-") do |lock_dir|
+        ENV['FLUENTD_LOCK_DIR'] = lock_dir
+        se.run
+      end
     end
 
     def install_main_process_signal_handlers
diff --git a/test/plugin/test_base.rb b/test/plugin/test_base.rb
index 51d55c1b69..b9567c9228 100644
--- a/test/plugin/test_base.rb
+++ b/test/plugin/test_base.rb
@@ -1,4 +1,5 @@
 require_relative '../helper'
+require 'tmpdir'
 require 'fluent/plugin/base'
 
 module FluentPluginBaseTest
@@ -112,4 +113,37 @@ class FluentPluginBaseTest::DummyPlugin2 < Fluent::Plugin::TestBase
     assert_equal 1, logger.logs.size
     assert{ logger.logs.first.include?("invalid byte sequence is replaced in ") }
   end
+
+  test 'generates worker lock path safely' do
+    Dir.mktmpdir("test-fluentd-lock-") do |lock_dir|
+      ENV['FLUENTD_LOCK_DIR'] = lock_dir
+      p = FluentPluginBaseTest::DummyPlugin.new
+      path = p.get_lock_path("Aa\\|=~/_123");
+
+      assert_equal lock_dir, File.dirname(path)
+      assert_equal "fluentd-Aa______123.lock", File.basename(path)
+    end
+  end
+
+  test 'can acquire inter-worker locking' do
+    Dir.mktmpdir("test-fluentd-lock-") do |lock_dir|
+      ENV['FLUENTD_LOCK_DIR'] = lock_dir
+      p = FluentPluginBaseTest::DummyPlugin.new
+      lock_path = p.get_lock_path("test_base")
+
+      p.acquire_worker_lock("test_base") do
+        # With LOCK_NB set, flock() returns `false` when the
+        # file is already locked.
+        File.open(lock_path, "w") do |f|
+          assert_equal false, f.flock(File::LOCK_EX|File::LOCK_NB)
+        end
+      end
+
+      # Lock should be release by now. In that case, flock
+      # must return 0.
+      File.open(lock_path, "w") do |f|
+        assert_equal 0, f.flock(File::LOCK_EX|File::LOCK_NB)
+      end
+    end
+  end
 end