diff --git a/lib/fluent/plugin/buf_file.rb b/lib/fluent/plugin/buf_file.rb
index d9f2bd2866..945b9682f0 100644
--- a/lib/fluent/plugin/buf_file.rb
+++ b/lib/fluent/plugin/buf_file.rb
@@ -122,14 +122,17 @@ def resume
           m = new_metadata() # this metadata will be overwritten by resuming .meta file content
                              # so it should not added into @metadata_list for now
           mode = Fluent::Plugin::Buffer::FileChunk.assume_chunk_state(path)
+          if mode == :unknown
+            log.debug "uknown state chunk found", path: path
+            next
+          end
+
           chunk = Fluent::Plugin::Buffer::FileChunk.new(m, path, mode) # file chunk resumes contents of metadata
           case chunk.state
           when :staged
             stage[chunk.metadata] = chunk
           when :queued
             queue << chunk
-          else
-            raise "BUG: unexpected chunk state '#{chunk.state}' for path '#{path}'"
           end
         end
 
diff --git a/lib/fluent/plugin/buffer.rb b/lib/fluent/plugin/buffer.rb
index 2670bbbaaf..1eff6de52f 100644
--- a/lib/fluent/plugin/buffer.rb
+++ b/lib/fluent/plugin/buffer.rb
@@ -104,11 +104,13 @@ def start
           @queued_num[chunk.metadata] += 1
           @queue_size += chunk.bytesize
         end
+        log.debug "buffer started", instance: self.object_id, stage_size: @stage_size, queue_size: @queue_size
       end
 
       def close
         super
         synchronize do
+          log.debug "closing buffer", instance: self.object_id
           @dequeued.each_pair do |chunk_id, chunk|
             chunk.close
           end
@@ -156,6 +158,7 @@ def new_metadata(timekey: nil, tag: nil, variables: nil)
       end
 
       def add_metadata(metadata)
+        log.trace "adding metadata", instance: self.object_id, metadata: metadata
         synchronize do
           if i = @metadata_list.index(metadata)
             @metadata_list[i]
@@ -178,6 +181,8 @@ def write(metadata_and_data, format: nil, size: nil, enqueue: false)
         return if metadata_and_data.size < 1
         raise BufferOverflowError, "buffer space has too many data" unless storable?
 
+        log.trace "writing events into buffer", instance: self.object_id, metadata_size: metadata_and_data.size
+
         staged_bytesize = 0
         operated_chunks = []
         unstaged_chunks = {} # metadata => [chunk, chunk, ...]
@@ -293,6 +298,7 @@ def queued?(metadata=nil)
       end
 
       def enqueue_chunk(metadata)
+        log.debug "enqueueing chunk", instance: self.object_id, metadata: metadata
         synchronize do
           chunk = @stage.delete(metadata)
           return nil unless chunk
@@ -314,6 +320,7 @@ def enqueue_chunk(metadata)
       end
 
       def enqueue_unstaged_chunk(chunk)
+        log.debug "enqueueing unstaged chunk", instance: self.object_id, metadata: chunk.metadata
         synchronize do
           chunk.synchronize do
             metadata = chunk.metadata
@@ -326,6 +333,7 @@ def enqueue_unstaged_chunk(chunk)
       end
 
       def enqueue_all
+        log.debug "enqueueing all chunks in buffer", instance: self.object_id
         synchronize do
           if block_given?
             @stage.keys.each do |metadata|
@@ -343,6 +351,7 @@ def enqueue_all
 
       def dequeue_chunk
         return nil if @queue.empty?
+        log.debug "dequeueing a chunk", instance: self.object_id
         synchronize do
           chunk = @queue.shift
 
@@ -351,15 +360,18 @@ def dequeue_chunk
 
           @dequeued[chunk.unique_id] = chunk
           @queued_num[chunk.metadata] -= 1 # BUG if nil, 0 or subzero
+          log.debug "chunk dequeued", instance: self.object_id, metadata: chunk.metadata
           chunk
         end
       end
 
       def takeback_chunk(chunk_id)
+        log.debug "taking back a chunk", instance: self.object_id, chunk_id: dump_unique_id_hex(chunk_id)
         synchronize do
           chunk = @dequeued.delete(chunk_id)
           return false unless chunk # already purged by other thread
           @queue.unshift(chunk)
+          log.debug "chunk taken back", instance: self.object_id, chunk_id: dump_unique_id_hex(chunk_id), metadata: chunk.metadata
           @queued_num[chunk.metadata] += 1 # BUG if nil
         end
         true
@@ -371,22 +383,26 @@ def purge_chunk(chunk_id)
           return nil unless chunk # purged by other threads
 
           metadata = chunk.metadata
+          log.debug "purging a chunk", instance: self.object_id, chunk_id: dump_unique_id_hex(chunk_id), metadata: metadata
           begin
             bytesize = chunk.bytesize
             chunk.purge
             @queue_size -= bytesize
           rescue => e
             log.error "failed to purge buffer chunk", chunk_id: dump_unique_id_hex(chunk_id), error_class: e.class, error: e
+            log.error_backtrace
           end
 
           if metadata && !@stage[metadata] && (!@queued_num[metadata] || @queued_num[metadata] < 1)
             @metadata_list.delete(metadata)
           end
+          log.debug "chunk purged", instance: self.object_id, chunk_id: dump_unique_id_hex(chunk_id), metadata: metadata
         end
         nil
       end
 
       def clear_queue!
+        log.debug "clearing queue", instance: self.object_id
         synchronize do
           until @queue.empty?
             begin
@@ -395,6 +411,7 @@ def clear_queue!
               q.purge
             rescue => e
               log.error "unexpected error while clearing buffer queue", error_class: e.class, error: e
+              log.error_backtrace
             end
           end
           @queue_size = 0
diff --git a/lib/fluent/plugin/buffer/file_chunk.rb b/lib/fluent/plugin/buffer/file_chunk.rb
index 09d9419700..6fda86da16 100644
--- a/lib/fluent/plugin/buffer/file_chunk.rb
+++ b/lib/fluent/plugin/buffer/file_chunk.rb
@@ -149,7 +149,9 @@ def self.assume_chunk_state(path)
           if /\.(b|q)([0-9a-f]+)\.[^\/]*\Z/n =~ path # //n switch means explicit 'ASCII-8BIT' pattern
             $1 == 'b' ? :staged : :queued
           else
-            :queued
+            # files which matches to glob of buffer file pattern
+            # it includes files which are created by out_file
+            :unknown
           end
         end
 
diff --git a/lib/fluent/plugin/output.rb b/lib/fluent/plugin/output.rb
index 5a19204275..32bc6ef3d0 100644
--- a/lib/fluent/plugin/output.rb
+++ b/lib/fluent/plugin/output.rb
@@ -695,6 +695,7 @@ def handle_stream_simple(tag, es, enqueue: false)
       end
 
       def commit_write(chunk_id, delayed: @delayed_commit, secondary: false)
+        log.trace "committing write operation to a chunk", chunk: dump_unique_id_hex(chunk_id), delayed: delayed
         if delayed
           @dequeued_chunks_mutex.synchronize do
             @dequeued_chunks.delete_if{ |info| info.chunk_id == chunk_id }
@@ -770,6 +771,8 @@ def try_flush
         chunk = @buffer.dequeue_chunk
         return unless chunk
 
+        log.debug "trying flush for a chunk", chunk: dump_unique_id_hex(chunk.unique_id)
+
         output = self
         using_secondary = false
         if @retry_mutex.synchronize{ @retry && @retry.secondary? }
@@ -783,6 +786,7 @@ def try_flush
 
         begin
           if output.delayed_commit
+            log.trace "executing delayed write and commit", chunk: dump_unique_id_hex(chunk.unique_id)
             @counters_monitor.synchronize{ @write_count += 1 }
             output.try_write(chunk)
             @dequeued_chunks_mutex.synchronize do
@@ -791,9 +795,14 @@ def try_flush
             end
           else # output plugin without delayed purge
             chunk_id = chunk.unique_id
+            dump_chunk_id = dump_unique_id_hex(chunk_id)
+            log.trace "adding write count", instance: self.object_id
             @counters_monitor.synchronize{ @write_count += 1 }
+            log.trace "executing sync write", chunk: dump_chunk_id
             output.write(chunk)
+            log.trace "write operation done, committing", chunk: dump_chunk_id
             commit_write(chunk_id, secondary: using_secondary)
+            log.trace "done to commit a chunk", chunk: dump_chunk_id
           end
         rescue => e
           log.debug "taking back chunk for errors.", plugin_id: plugin_id, chunk: dump_unique_id_hex(chunk.unique_id)
diff --git a/test/plugin/test_buf_file.rb b/test/plugin/test_buf_file.rb
index 21cd7ef68f..aeb3257941 100644
--- a/test/plugin/test_buf_file.rb
+++ b/test/plugin/test_buf_file.rb
@@ -500,4 +500,72 @@ def write_metadata(path, chunk_id, metadata, size, ctime, mtime)
       assert_equal Time.parse('2016-04-17 14:01:22 -0700'), queue[3].modified_at
     end
   end
+
+  sub_test_case 'there are some non-buffer chunk files, with a path without buffer chunk ids' do
+    setup do
+      @bufdir = File.expand_path('../../tmp/buffer_file', __FILE__)
+
+      @c1id = Fluent::UniqueId.generate
+      p1 = File.join(@bufdir, "etest.201604171358.q#{Fluent::UniqueId.hex(@c1id)}.log")
+      File.open(p1, 'wb') do |f|
+        f.write ["t1.test", event_time('2016-04-17 13:58:15 -0700').to_i, {"message" => "yay"}].to_json + "\n"
+        f.write ["t2.test", event_time('2016-04-17 13:58:17 -0700').to_i, {"message" => "yay"}].to_json + "\n"
+        f.write ["t3.test", event_time('2016-04-17 13:58:21 -0700').to_i, {"message" => "yay"}].to_json + "\n"
+        f.write ["t4.test", event_time('2016-04-17 13:58:22 -0700').to_i, {"message" => "yay"}].to_json + "\n"
+      end
+      FileUtils.touch(p1, mtime: Time.parse('2016-04-17 13:58:28 -0700'))
+
+      @not_chunk = File.join(@bufdir, 'etest.20160416.log')
+      File.open(@not_chunk, 'wb') do |f|
+        f.write ["t1.test", event_time('2016-04-16 23:58:15 -0700').to_i, {"message" => "yay"}].to_json + "\n"
+        f.write ["t2.test", event_time('2016-04-16 23:58:17 -0700').to_i, {"message" => "yay"}].to_json + "\n"
+        f.write ["t3.test", event_time('2016-04-16 23:58:21 -0700').to_i, {"message" => "yay"}].to_json + "\n"
+        f.write ["t4.test", event_time('2016-04-16 23:58:22 -0700').to_i, {"message" => "yay"}].to_json + "\n"
+      end
+      FileUtils.touch(@not_chunk, mtime: Time.parse('2016-04-17 00:00:00 -0700'))
+
+      @bufpath = File.join(@bufdir, 'etest.*.log')
+
+      Fluent::Test.setup
+      @d = FluentPluginFileBufferTest::DummyOutputPlugin.new
+      @p = Fluent::Plugin::FileBuffer.new
+      @p.owner = @d
+      @p.configure(config_element('buffer', '', {'path' => @bufpath}))
+      @p.start
+    end
+
+    teardown do
+      if @p
+        @p.stop unless @p.stopped?
+        @p.before_shutdown unless @p.before_shutdown?
+        @p.shutdown unless @p.shutdown?
+        @p.after_shutdown unless @p.after_shutdown?
+        @p.close unless @p.closed?
+        @p.terminate unless @p.terminated?
+      end
+      if @bufdir
+        Dir.glob(File.join(@bufdir, '*')).each do |path|
+          next if ['.', '..'].include?(File.basename(path))
+          File.delete(path)
+        end
+      end
+    end
+
+    test '#resume returns queued chunks for files without metadata, while ignoring non-chunk looking files' do
+      assert_equal 0, @p.stage.size
+      assert_equal 1, @p.queue.size
+
+      queue = @p.queue
+
+      m = metadata()
+
+      assert_equal @c1id, queue[0].unique_id
+      assert_equal m, queue[0].metadata
+      assert_equal 0, queue[0].size
+      assert_equal :queued, queue[0].state
+      assert_equal Time.parse('2016-04-17 13:58:28 -0700'), queue[0].modified_at
+
+      assert File.exist?(@not_chunk)
+    end
+  end
 end
diff --git a/test/plugin/test_buffer_file_chunk.rb b/test/plugin/test_buffer_file_chunk.rb
index 293060b8e9..13e72f8b68 100644
--- a/test/plugin/test_buffer_file_chunk.rb
+++ b/test/plugin/test_buffer_file_chunk.rb
@@ -49,8 +49,9 @@ def hex_id(id)
     data(
       correct_staged: ['/mydir/mypath/myfile.b00ff.log', :staged],
       correct_queued: ['/mydir/mypath/myfile.q00ff.log', :queued],
-      incorrect_staged: ['/mydir/mypath/myfile.b00ff.log/unknown', :queued],
-      incorrect_queued: ['/mydir/mypath/myfile.q00ff.log/unknown', :queued],
+      incorrect_staged: ['/mydir/mypath/myfile.b00ff.log/unknown', :unknown],
+      incorrect_queued: ['/mydir/mypath/myfile.q00ff.log/unknown', :unknown],
+      output_file: ['/mydir/mypath/myfile.20160716.log', :unknown],
     )
     test 'can .assume_chunk_state' do |data|
       path, expected = data