diff --git a/lib/fluent/plugin/buffer.rb b/lib/fluent/plugin/buffer.rb
index 58154a9d93..c39c17c109 100644
--- a/lib/fluent/plugin/buffer.rb
+++ b/lib/fluent/plugin/buffer.rb
@@ -372,7 +372,7 @@ def write(metadata_and_data, format: nil, size: nil, enqueue: false)
       end
 
       def queue_full?
-        @queued_chunks_limit_size && (synchronize { @queue.size } >= @queued_chunks_limit_size)
+        synchronize { @queue.size } >= @queued_chunks_limit_size
       end
 
       def queued_records
diff --git a/lib/fluent/plugin/output.rb b/lib/fluent/plugin/output.rb
index 85a5593ebf..66077254c9 100644
--- a/lib/fluent/plugin/output.rb
+++ b/lib/fluent/plugin/output.rb
@@ -354,6 +354,10 @@ def configure(conf)
               log.warn "'flush_interval' is ignored because default 'flush_mode' is not 'interval': '#{@flush_mode}'"
             end
           end
+
+          if @buffer.queued_chunks_limit_size.nil?
+            @buffer.queued_chunks_limit_size = @buffer_config.flush_thread_count
+          end
         end
 
         if @secondary_config
diff --git a/test/plugin/test_buffer.rb b/test/plugin/test_buffer.rb
index 728bc9b84d..88e4b631be 100644
--- a/test/plugin/test_buffer.rb
+++ b/test/plugin/test_buffer.rb
@@ -170,7 +170,7 @@ def create_chunk_es(metadata, es)
 
   sub_test_case 'with default configuration and dummy implementation' do
     setup do
-      @p = create_buffer({})
+      @p = create_buffer({'queued_chunks_limit_size' => 100})
       @dm0 = create_metadata(Time.parse('2016-04-11 16:00:00 +0000').to_i, nil, nil)
       @dm1 = create_metadata(Time.parse('2016-04-11 16:10:00 +0000').to_i, nil, nil)
       @dm2 = create_metadata(Time.parse('2016-04-11 16:20:00 +0000').to_i, nil, nil)
diff --git a/test/plugin/test_output_as_buffered.rb b/test/plugin/test_output_as_buffered.rb
index 2f94d35a13..ca3552b4d2 100644
--- a/test/plugin/test_output_as_buffered.rb
+++ b/test/plugin/test_output_as_buffered.rb
@@ -219,6 +219,24 @@ def waiting(seconds)
     Timecop.return
   end
 
+  test 'queued_chunks_limit_size is same as flush_thread_count by default' do
+    hash = {'flush_thread_count' => 4}
+    i = create_output
+    i.register(:prefer_buffered_processing) { true }
+    i.configure(config_element('ROOT', '', {}, [config_element('buffer','tag',hash)]))
+
+    assert_equal 4, i.buffer.queued_chunks_limit_size
+  end
+
+  test 'prefer queued_chunks_limit_size parameter than flush_thread_count' do
+    hash = {'flush_thread_count' => 4, 'queued_chunks_limit_size' => 2}
+    i = create_output
+    i.register(:prefer_buffered_processing) { true }
+    i.configure(config_element('ROOT', '', {}, [config_element('buffer','tag',hash)]))
+
+    assert_equal 2, i.buffer.queued_chunks_limit_size
+  end
+
   sub_test_case 'chunk feature in #write for output plugins' do
     setup do
       @stored_global_logger = $log
@@ -1064,6 +1082,7 @@ def waiting(seconds)
         'flush_thread_count' => 1,
         'flush_thread_burst_interval' => 0.1,
         'chunk_limit_size' => 1024,
+        'queued_chunks_limit_size' => 100
       }
       @i = create_output(:buffered)
       @i.configure(config_element('ROOT','',{},[config_element('buffer',chunk_key,hash)]))
diff --git a/test/plugin/test_output_as_buffered_retries.rb b/test/plugin/test_output_as_buffered_retries.rb
index e7d74bdc49..7d76cd3bba 100644
--- a/test/plugin/test_output_as_buffered_retries.rb
+++ b/test/plugin/test_output_as_buffered_retries.rb
@@ -123,6 +123,7 @@ def get_log_time(msg, logs)
         'flush_interval' => 1,
         'flush_thread_burst_interval' => 0.1,
         'retry_randomize' => false,
+        'queued_chunks_limit_size' => 100
       }
       @i.configure(config_element('ROOT','',{},[config_element('buffer',chunk_key,hash)]))
       @i.register(:prefer_buffered_processing){ true }
@@ -252,6 +253,7 @@ def get_log_time(msg, logs)
         'flush_thread_burst_interval' => 0.1,
         'retry_randomize' => false,
         'retry_timeout' => 3600,
+        'queued_chunks_limit_size' => 100
       }
       @i.configure(config_element('ROOT','',{},[config_element('buffer',chunk_key,hash)]))
       @i.register(:prefer_buffered_processing){ true }
@@ -342,6 +344,7 @@ def get_log_time(msg, logs)
         'flush_thread_burst_interval' => 0.1,
         'retry_randomize' => false,
         'retry_max_times' => 10,
+        'queued_chunks_limit_size' => 100
       }
       @i.configure(config_element('ROOT','',{},[config_element('buffer',chunk_key,hash)]))
       @i.register(:prefer_buffered_processing){ true }
@@ -485,6 +488,7 @@ def get_log_time(msg, logs)
         'retry_type' => :periodic,
         'retry_wait' => 3,
         'retry_randomize' => false,
+        'queued_chunks_limit_size' => 100
       }
       @i.configure(config_element('ROOT','',{},[config_element('buffer',chunk_key,hash)]))
       @i.register(:prefer_buffered_processing){ true }
@@ -531,6 +535,7 @@ def get_log_time(msg, logs)
         'retry_wait' => 30,
         'retry_randomize' => false,
         'retry_timeout' => 120,
+        'queued_chunks_limit_size' => 100
       }
       @i.configure(config_element('ROOT','',{},[config_element('buffer',chunk_key,hash)]))
       @i.register(:prefer_buffered_processing){ true }
@@ -626,6 +631,7 @@ def get_log_time(msg, logs)
         'retry_wait' => 3,
         'retry_randomize' => false,
         'retry_max_times' => 10,
+        'queued_chunks_limit_size' => 100
       }
       @i.configure(config_element('ROOT','',{},[config_element('buffer',chunk_key,hash)]))
       @i.register(:prefer_buffered_processing){ true }
@@ -724,6 +730,7 @@ def get_log_time(msg, logs)
         'retry_randomize' => false,
         'retry_timeout' => 3600,
         'retry_max_times' => 10,
+        'queued_chunks_limit_size' => 100
       }
       @i.configure(config_element('ROOT','',{},[config_element('buffer',chunk_key,hash)]))
       @i.register(:prefer_buffered_processing){ true }
@@ -795,6 +802,7 @@ def get_log_time(msg, logs)
         'retry_wait' => 30,
         'retry_timeout' => 360,
         'retry_max_times' => 10,
+        'queued_chunks_limit_size' => 100
       }
       @i.configure(config_element('ROOT','',{},[config_element('buffer',chunk_key,hash)]))
       @i.register(:prefer_buffered_processing){ true }
diff --git a/test/plugin/test_output_as_buffered_secondary.rb b/test/plugin/test_output_as_buffered_secondary.rb
index a98f0c68bf..d1b398f7b6 100644
--- a/test/plugin/test_output_as_buffered_secondary.rb
+++ b/test/plugin/test_output_as_buffered_secondary.rb
@@ -544,7 +544,7 @@ def dummy_event_stream
     test 'secondary plugin can do delayed commit even if primary does not do it, and non-committed chunks will be rollbacked by primary' do
       written = []
       chunks = []
-      priconf = config_element('buffer','tag',{'flush_interval' => 1, 'retry_type' => :periodic, 'retry_wait' => 3, 'retry_timeout' => 60, 'delayed_commit_timeout' => 2, 'retry_randomize' => false})
+      priconf = config_element('buffer','tag',{'flush_interval' => 1, 'retry_type' => :periodic, 'retry_wait' => 3, 'retry_timeout' => 60, 'delayed_commit_timeout' => 2, 'retry_randomize' => false, 'queued_chunks_limit_size' => 10})
       secconf = config_element('secondary','',{'@type' => 'output_secondary_test2'})
       @i.configure(config_element('ROOT','',{},[priconf,secconf]))
       @i.register(:prefer_buffered_processing){ true }