diff --git a/lib/fluent/plugin/filter_concat.rb b/lib/fluent/plugin/filter_concat.rb
index f990aa7..6510ce3 100644
--- a/lib/fluent/plugin/filter_concat.rb
+++ b/lib/fluent/plugin/filter_concat.rb
@@ -35,6 +35,7 @@ def initialize
 
       @buffer = Hash.new {|h, k| h[k] = [] }
       @timeout_map = Hash.new {|h, k| h[k] = Fluent::Engine.now }
+      @mutex = Thread::Mutex.new
     end
 
     def configure(conf)
@@ -122,9 +123,13 @@ def process(tag, time, record)
 
     def process_line(stream_identity, tag, time, record)
       new_es = Fluent::MultiEventStream.new
-      @buffer[stream_identity] << [tag, time, record]
+      @mutex.synchronize do
+        @buffer[stream_identity] << [tag, time, record]
+      end
       if @buffer[stream_identity].size >= @n_lines
-        new_time, new_record = flush_buffer(stream_identity)
+        new_time, new_record = @mutex.synchronize do
+          flush_buffer(stream_identity)
+        end
         time = new_time if @use_first_timestamp
         new_es.add(time, new_record)
       end
@@ -136,26 +141,36 @@ def process_regexp(stream_identity, tag, time, record)
       case
       when firstline?(record[@key])
         if @buffer[stream_identity].empty?
-          @buffer[stream_identity] << [tag, time, record]
+          @mutex.synchronize do
+            @buffer[stream_identity] << [tag, time, record]
+          end
           if lastline?(record[@key])
-            new_time, new_record = flush_buffer(stream_identity)
+            new_time, new_record = @mutex.synchronize do
+              flush_buffer(stream_identity)
+            end
             time = new_time if @use_first_timestamp
             new_es.add(time, new_record)
           end
         else
-          new_time, new_record = flush_buffer(stream_identity, [tag, time, record])
+          new_time, new_record = @mutex.synchronize do
+            flush_buffer(stream_identity, [tag, time, record])
+          end
           time = new_time if @use_first_timestamp
           new_es.add(time, new_record)
           if lastline?(record[@key])
-            new_time, new_record = flush_buffer(stream_identity)
+            new_time, new_record = @mutex.synchronize do
+              flush_buffer(stream_identity)
+            end
             time = new_time if @use_first_timestamp
             new_es.add(time, new_record)
           end
           return new_es
         end
       when lastline?(record[@key])
-        @buffer[stream_identity] << [tag, time, record]
-        new_time, new_record = flush_buffer(stream_identity)
+        @mutex.synchronize do
+          @buffer[stream_identity] << [tag, time, record]
+          new_time, new_record = flush_buffer(stream_identity)
+        end
         time = new_time if @use_first_timestamp
         new_es.add(time, new_record)
         return new_es
@@ -166,9 +181,13 @@ def process_regexp(stream_identity, tag, time, record)
         else
           if continuous_line?(record[@key])
             # Continuation of the previous line
-            @buffer[stream_identity] << [tag, time, record]
+            @mutex.synchronize do
+              @buffer[stream_identity] << [tag, time, record]
+            end
           else
-            new_time, new_record = flush_buffer(stream_identity)
+            new_time, new_record = @mutex.synchronize do
+              flush_buffer(stream_identity)
+            end
             time = new_time if @use_first_timestamp
             new_es.add(time, new_record)
             new_es.add(time, record)
@@ -211,7 +230,10 @@ def flush_timeout_buffer
       @timeout_map.each do |stream_identity, previous_timestamp|
         next if @flush_interval > (now - previous_timestamp)
         next if @buffer[stream_identity].empty?
-        time, flushed_record = flush_buffer(stream_identity)
+        next if @mutex.locked?
+        time, flushed_record = @mutex.synchronize do
+          flush_buffer(stream_identity)
+        end
         timeout_stream_identities << stream_identity
         tag = stream_identity.split(":").first
         message = "Timeout flush: #{stream_identity}"