diff --git a/lib/fluent/plugin/in_tcp.rb b/lib/fluent/plugin/in_tcp.rb
index 428eb5258a..71d77b0f4f 100644
--- a/lib/fluent/plugin/in_tcp.rb
+++ b/lib/fluent/plugin/in_tcp.rb
@@ -55,13 +55,12 @@ def multi_workers_ready?
     def start
       super
 
-      @buffer = ''
       server_create(:in_tcp_server, @port, bind: @bind) do |data, conn|
-        @buffer << data
+        conn.buffer << data
         begin
           pos = 0
-          while i = @buffer.index(@delimiter, pos)
-            msg = @buffer[pos...i]
+          while i = conn.buffer.index(@delimiter, pos)
+            msg = conn.buffer[pos...i]
             pos = i + @delimiter.length
 
             @parser.parse(msg) do |time, record|
@@ -77,7 +76,7 @@ def start
               router.emit(tag, time, record)
             end
           end
-          @buffer.slice!(0, pos) if pos > 0
+          conn.buffer.slice!(0, pos) if pos > 0
         end
       end
     end
diff --git a/lib/fluent/plugin_helper/server.rb b/lib/fluent/plugin_helper/server.rb
index ffdb57dab1..eb57832475 100644
--- a/lib/fluent/plugin_helper/server.rb
+++ b/lib/fluent/plugin_helper/server.rb
@@ -425,9 +425,12 @@ def on(event, &callback)
       end
 
       class TCPCallbackSocket < CallbackSocket
+        attr_accessor :buffer
+
         def initialize(sock)
           super("tcp", sock, [:data, :write_complete, :close])
           @peeraddr = @sock.peeraddr
+          @buffer = ''
         end
 
         def write(data)