diff --git a/lib/fluent/plugin/in_tail.rb b/lib/fluent/plugin/in_tail.rb
index 0baf9d8a79..dbe628bea7 100644
--- a/lib/fluent/plugin/in_tail.rb
+++ b/lib/fluent/plugin/in_tail.rb
@@ -869,8 +869,9 @@ def reset_timer
     class PositionFile
       UNWATCHED_POSITION = 0xffffffffffffffff
 
-      def initialize(file, map, last_pos)
+      def initialize(file, file_mutex, map, last_pos)
         @file = file
+        @file_mutex = file_mutex
         @map = map
         @last_pos = last_pos
       end
@@ -880,31 +881,34 @@ def [](path)
           return m
         end
 
-        @file.pos = @last_pos
-        @file.write path
-        @file.write "\t"
-        seek = @file.pos
-        @file.write "0000000000000000\t0000000000000000\n"
-        @last_pos = @file.pos
-
-        @map[path] = FilePositionEntry.new(@file, seek, 0, 0)
+        @file_mutex.synchronize {
+          @file.pos = @last_pos
+          @file.write "#{path}\t0000000000000000\t0000000000000000\n"
+          seek = @last_pos + path.bytesize + 1
+          @last_pos = @file.pos
+          @map[path] = FilePositionEntry.new(@file, @file_mutex, seek, 0, 0)
+        }
       end
 
       def self.parse(file)
         compact(file)
 
+        file_mutex = Mutex.new
         map = {}
         file.pos = 0
         file.each_line {|line|
           m = /^([^\t]+)\t([0-9a-fA-F]+)\t([0-9a-fA-F]+)/.match(line)
-          next unless m
+          unless m
+            $log.warn "Unparsable line in pos_file: #{line}"
+            next
+          end
           path = m[1]
           pos = m[2].to_i(16)
           ino = m[3].to_i(16)
           seek = file.pos - line.bytesize + path.bytesize + 1
-          map[path] = FilePositionEntry.new(file, seek, pos, ino)
+          map[path] = FilePositionEntry.new(file, file_mutex, seek, pos, ino)
         }
-        new(file, map, file.pos)
+        new(file, file_mutex, map, file.pos)
       end
 
       # Clean up unwatched file entries
@@ -912,7 +916,10 @@ 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)
-          next unless m
+          unless m
+            $log.warn "Unparsable line in pos_file: #{line}"
+            next
+          end
           path = m[1]
           pos = m[2].to_i(16)
           ino = m[3].to_i(16)
@@ -935,23 +942,28 @@ class FilePositionEntry
       LN_OFFSET = 33
       SIZE = 34
 
-      def initialize(file, seek, pos, inode)
+      def initialize(file, file_mutex, seek, pos, inode)
         @file = file
+        @file_mutex = file_mutex
         @seek = seek
         @pos = pos 
         @inode = inode
       end
 
       def update(ino, pos)
-        @file.pos = @seek
-        @file.write "%016x\t%016x" % [pos, ino]
+        @file_mutex.synchronize {
+          @file.pos = @seek
+          @file.write "%016x\t%016x" % [pos, ino]
+        }
         @pos = pos
         @inode = ino
       end
 
       def update_pos(pos)
-        @file.pos = @seek
-        @file.write "%016x" % pos
+        @file_mutex.synchronize {
+          @file.pos = @seek
+          @file.write "%016x" % pos
+        }
         @pos = pos
       end