diff --git a/lib/fluent/plugin/in_tail.rb b/lib/fluent/plugin/in_tail.rb
index 6a226a544d..7e46f3d729 100644
--- a/lib/fluent/plugin/in_tail.rb
+++ b/lib/fluent/plugin/in_tail.rb
@@ -307,7 +307,7 @@ def start_watchers(paths)
           pe = @pf[path]
           if @read_from_head && pe.read_inode.zero?
             begin
-              pe.update(Fluent::FileWrapper.stat(path).ino, 0)
+              pe.update(Fluent::FileWrapper.stat(path).ino, 0, 0)
             rescue Errno::ENOENT
               $log.warn "#{path} not found. Continuing without tailing it."
             end
@@ -372,7 +372,7 @@ def detach_watcher(tw, close_io = true)
       tw.close if close_io
       flush_buffer(tw)
       if tw.unwatched && @pf
-        @pf[tw.path].update_pos(PositionFile::UNWATCHED_POSITION)
+        @pf[tw.path].update_pos(PositionFile::UNWATCHED_POSITION, 0)
       end
     end
 
@@ -580,19 +580,19 @@ def on_rotate(stat)
               # in either case of a and b, seek to the saved position
               #   c) file was once renamed, truncated and then backed
               # in this case, consider it truncated
-              @pe.update(inode, 0) if fsize < @pe.read_pos
+              @pe.update(inode, 0, fsize) if fsize < @pe.read_pos
             elsif last_inode != 0
               # this is FilePositionEntry and fluentd once started.
               # read data from the head of the rotated file.
               # logs never duplicate because this file is a rotated new file.
-              @pe.update(inode, 0)
+              @pe.update(inode, 0, fize)
             else
               # this is MemoryPositionEntry or this is the first time fluentd started.
               # seek to the end of the any files.
               # logs may duplicate without this seek because it's not sure the file is
               # existent file or rotated new file.
               pos = @read_from_head ? 0 : fsize
-              @pe.update(inode, pos)
+              @pe.update(inode, pos, fsize)
             end
             @io_handler = IOHandler.new(self, &method(:wrap_receive_lines))
           else
@@ -604,10 +604,10 @@ def on_rotate(stat)
           if stat
             inode = stat.ino
             if inode == @pe.read_inode # truncated
-              @pe.update_pos(0)
+              @pe.update_pos(0, stat.size)
               @io_handler.close
             elsif !@io_handler.opened? # There is no previous file. Reuse TailWatcher
-              @pe.update(inode, 0)
+              @pe.update(inode, 0, stat.size)
             else # file is rotated and new file found
               watcher_needs_update = true
               # Handle the old log file before renewing TailWatcher [fluentd#1055]
@@ -634,7 +634,7 @@ def on_rotate(stat)
       def swap_state(pe)
         # Use MemoryPositionEntry for rotated file temporary
         mpe = MemoryPositionEntry.new
-        mpe.update(pe.read_inode, pe.read_pos)
+        mpe.update(pe.read_inode, pe.read_pos, pe.read_size)
         @pe = mpe
         pe # This pe will be updated in on_rotate after TailWatcher is initialized
       end
@@ -766,7 +766,7 @@ def handle_notify
 
               unless @lines.empty?
                 if @receive_lines.call(@lines)
-                  @watcher.pe.update_pos(io.pos - @fifo.bytesize)
+                  @watcher.pe.update_pos(io.pos - @fifo.bytesize, io.size)
                   @lines.clear
                 else
                   read_more = false
@@ -912,10 +912,10 @@ def [](path)
 
         @file_mutex.synchronize {
           @file.pos = @last_pos
-          @file.write "#{path}\t0000000000000000\t0000000000000000\n"
+          @file.write "#{path}\t0000000000000000\t0000000000000000\t0000000000000000\n"
           seek = @last_pos + path.bytesize + 1
           @last_pos = @file.pos
-          @map[path] = FilePositionEntry.new(@file, @file_mutex, seek, 0, 0)
+          @map[path] = FilePositionEntry.new(@file, @file_mutex, seek, 0, 0, 0)
         }
       end
 
@@ -926,7 +926,7 @@ def self.parse(file)
         map = {}
         file.pos = 0
         file.each_line {|line|
-          m = /^([^\t]+)\t([0-9a-fA-F]+)\t([0-9a-fA-F]+)/.match(line)
+          m = /^([^\t]+)\t([0-9a-fA-F]+)\t([0-9a-fA-F]+)[\t]?([0-9a-fA-F]+)?/.match(line)
           unless m
             $log.warn "Unparsable line in pos_file: #{line}"
             next
@@ -934,8 +934,9 @@ def self.parse(file)
           path = m[1]
           pos = m[2].to_i(16)
           ino = m[3].to_i(16)
+          size = m[4].to_i(16)
           seek = file.pos - line.bytesize + path.bytesize + 1
-          map[path] = FilePositionEntry.new(file, file_mutex, seek, pos, ino)
+          map[path] = FilePositionEntry.new(file, file_mutex, seek, pos, size, ino)
         }
         new(file, file_mutex, map, file.pos)
       end
@@ -944,7 +945,7 @@ def self.parse(file)
       def self.compact(file)
         file.pos = 0
         existent_entries = file.each_line.map { |line|
-          m = /^([^\t]+)\t([0-9a-fA-F]+)\t([0-9a-fA-F]+)/.match(line)
+          m = /^([^\t]+)\t([0-9a-fA-F]+)\t([0-9a-fA-F]+)[\t]?([0-9a-fA-F]+)?/.match(line)
           unless m
             $log.warn "Unparsable line in pos_file: #{line}"
             next
@@ -952,8 +953,9 @@ def self.compact(file)
           path = m[1]
           pos = m[2].to_i(16)
           ino = m[3].to_i(16)
+          m[4].nil? ? size = 0 : size = m[4].to_i(16)
           # 32bit inode converted to 64bit at this phase
-          pos == UNWATCHED_POSITION ? nil : ("%s\t%016x\t%016x\n" % [path, pos, ino])
+          pos == UNWATCHED_POSITION ? nil : ("%s\t%016x\t%016x\t%016x\n" % [path, pos, ino, size])
         }.compact
 
         file.pos = 0
@@ -971,35 +973,42 @@ class FilePositionEntry
       LN_OFFSET = 33
       SIZE = 34
 
-      def initialize(file, file_mutex, seek, pos, inode)
+      def initialize(file, file_mutex, seek, pos, size, inode)
         @file = file
         @file_mutex = file_mutex
         @seek = seek
         @pos = pos
+        @size = size
         @inode = inode
       end
 
-      def update(ino, pos)
+      def update(ino, pos, size)
         @file_mutex.synchronize {
           @file.pos = @seek
-          @file.write "%016x\t%016x" % [pos, ino]
+          @file.write "%016x\t%016x\t%016x" % [pos, ino, size]
         }
         @pos = pos
         @inode = ino
+        @size = size
       end
 
-      def update_pos(pos)
+      def update_pos(pos, size)
         @file_mutex.synchronize {
           @file.pos = @seek
-          @file.write "%016x" % pos
+          @file.write "%016x\t%016x\t%016x" % [pos, @inode, size]
         }
         @pos = pos
+        @size = size
       end
 
       def read_inode
         @inode
       end
 
+      def read_size
+        @size
+      end
+
       def read_pos
         @pos
       end
@@ -1009,21 +1018,28 @@ class MemoryPositionEntry
       def initialize
         @pos = 0
         @inode = 0
+        @size = 0
       end
 
-      def update(ino, pos)
+      def update(ino, pos, size)
         @inode = ino
         @pos = pos
+        @size = size
       end
 
-      def update_pos(pos)
+      def update_pos(pos, size)
         @pos = pos
+        @size = size
       end
 
       def read_pos
         @pos
       end
 
+      def read_size
+        @size
+      end
+
       def read_inode
         @inode
       end
diff --git a/test/plugin/test_in_tail.rb b/test/plugin/test_in_tail.rb
index d67cbdce3c..8d506bf40d 100644
--- a/test/plugin/test_in_tail.rb
+++ b/test/plugin/test_in_tail.rb
@@ -1050,6 +1050,32 @@ def test_pos_file_dir_creation
     d.instance_shutdown
   end
 
+  def test_pos_file_legacy_file_format_migrate_to_new_pos_file_format
+    config = config_element("", "", {
+      "tag" => "tail",
+      "path" => "#{TMP_DIR}/*.txt",
+      "format" => "none",
+      "pos_file" => "#{TMP_DIR}/pos/tail.pos",
+      "read_from_head" => true,
+      "refresh_interval" => 1
+    })
+    d = create_driver(config, false)
+    d.run(expect_emits: 1, shutdown: false) do
+      File.open("#{TMP_DIR}/pos/tail.pos", "wb") { |f| 
+        f.puts "#{TMP_DIR}/tail.txt\t0000000000000000\t0000000000000000\n" 
+      }
+      File.open("#{TMP_DIR}/tail.txt", "ab") { |f| f.puts "test3\n" }
+    end
+    assert_path_exist("#{TMP_DIR}/pos/tail.pos")
+
+    first_line_pos_file = ''
+    File.open("#{TMP_DIR}/pos/tail.pos", "r") { |f| first_line_pos_file = f.readline }
+    assert_equal(5, /^([^\t]+)\t([0-9a-fA-F]+)\t([0-9a-fA-F]+)\t([0-9a-fA-F]+)/.match(first_line_pos_file).length)
+
+    cleanup_directory(TMP_DIR)
+    d.instance_shutdown
+  end
+
   def test_z_refresh_watchers
     plugin = create_driver(EX_CONFIG, false).instance
     sio = StringIO.new