diff --git a/lib/fluent/plugin/in_tail.rb b/lib/fluent/plugin/in_tail.rb
index dd94560d0b..8861cc015e 100644
--- a/lib/fluent/plugin/in_tail.rb
+++ b/lib/fluent/plugin/in_tail.rb
@@ -1012,6 +1012,7 @@ def initialize(from_encoding, encoding, log, max_line_size=nil)
           @eol = "\n".encode(from_encoding).freeze
           @max_line_size = max_line_size
           @skip_current_line = false
+          @skipping_current_line_bytesize = 0
           @log = log
         end
 
@@ -1064,8 +1065,9 @@ def read_lines(lines)
             if is_long_line
               @log.warn "received line length is longer than #{@max_line_size}"
               @log.debug("skipped line: ") { convert(rbuf).chomp }
-              @skip_current_line = false
               has_skipped_line = true
+              @skip_current_line = false
+              @skipping_current_line_bytesize = 0
               next
             end
 
@@ -1077,14 +1079,16 @@ def read_lines(lines)
           )
 
           if is_long_current_line
-            @buffer.clear
             @skip_current_line = true
+            @skipping_current_line_bytesize += @buffer.bytesize
+            @buffer.clear
           end
 
           return has_skipped_line
         end
 
-        def bytesize
+        def reading_bytesize
+          return @skipping_current_line_bytesize if @skip_current_line
           @buffer.bytesize
         end
       end
@@ -1223,10 +1227,10 @@ def handle_notify
               end
 
               if @lines.empty?
-                @watcher.pe.update_pos(io.pos - @fifo.bytesize) if has_skipped_line
+                @watcher.pe.update_pos(io.pos - @fifo.reading_bytesize) if has_skipped_line
               else
                 if @receive_lines.call(@lines, @watcher)
-                  @watcher.pe.update_pos(io.pos - @fifo.bytesize)
+                  @watcher.pe.update_pos(io.pos - @fifo.reading_bytesize)
                   @lines.clear
                 else
                   read_more = false
@@ -1238,12 +1242,12 @@ def handle_notify
 
         def open
           io = Fluent::FileWrapper.open(@path)
-          io.seek(@watcher.pe.read_pos + @fifo.bytesize)
+          io.seek(@watcher.pe.read_pos + @fifo.reading_bytesize)
           @metrics.opened.inc
           io
         rescue RangeError
           io.close if io
-          raise WatcherSetupError, "seek error with #{@path}: file position = #{@watcher.pe.read_pos.to_s(16)}, reading bytesize = #{@fifo.bytesize.to_s(16)}"
+          raise WatcherSetupError, "seek error with #{@path}: file position = #{@watcher.pe.read_pos.to_s(16)}, reading bytesize = #{@fifo.reading_bytesize.to_s(16)}"
         rescue Errno::EACCES => e
           @log.warn "#{e}"
           nil
diff --git a/test/plugin/in_tail/test_fifo.rb b/test/plugin/in_tail/test_fifo.rb
index ebe0729d49..ebead83b92 100644
--- a/test/plugin/in_tail/test_fifo.rb
+++ b/test/plugin/in_tail/test_fifo.rb
@@ -124,7 +124,7 @@ class IntailFIFO < Test::Unit::TestCase
         fifo << text.force_encoding(Encoding::ASCII_8BIT)
         fifo.read_lines(lines)
         # The size of remaining buffer (i.e. a line still not having EOL) must not exceed max_line_size.
-        assert { fifo.bytesize <= max_line_size }
+        assert { fifo.buffer.bytesize <= max_line_size }
       end
 
       assert_equal expected, lines
@@ -142,23 +142,50 @@ class IntailFIFO < Test::Unit::TestCase
     end
   end
 
-  sub_test_case '#bytesize' do
-    test 'reutrns buffer size' do
+  sub_test_case '#reading_bytesize' do
+    test 'returns buffer size' do
       fifo = Fluent::Plugin::TailInput::TailWatcher::FIFO.new(Encoding::ASCII_8BIT, Encoding::ASCII_8BIT, $log)
       text = "test\n" * 3 + 'test'
       fifo << text
 
-      assert_equal text.bytesize, fifo.bytesize
+      assert_equal text.bytesize, fifo.reading_bytesize
       lines = []
       fifo.read_lines(lines)
       assert_equal ["test\n", "test\n", "test\n"], lines
 
-      assert_equal 'test'.bytesize, fifo.bytesize
+      assert_equal 'test'.bytesize, fifo.reading_bytesize
       fifo << "2\n"
       fifo.read_lines(lines)
       assert_equal ["test\n", "test\n", "test\n", "test2\n"], lines
 
-      assert_equal 0, fifo.bytesize
+      assert_equal 0, fifo.reading_bytesize
+    end
+
+    test 'returns the entire line size even if the size is over max_line_size' do
+      max_line_size = 20
+      fifo = Fluent::Plugin::TailInput::TailWatcher::FIFO.new(Encoding::ASCII_8BIT, Encoding::ASCII_8BIT, $log, max_line_size)
+      lines = []
+
+      text = "long line still not having EOL"
+      fifo << text
+      fifo.read_lines(lines)
+      assert_equal [], lines
+      assert_equal 0, fifo.buffer.bytesize
+      assert_equal text.bytesize, fifo.reading_bytesize
+
+      text2 = " following texts"
+      fifo << text2
+      fifo.read_lines(lines)
+      assert_equal [], lines
+      assert_equal 0, fifo.buffer.bytesize
+      assert_equal text.bytesize + text2.bytesize, fifo.reading_bytesize
+
+      text3 = " end of the line\n"
+      fifo << text3
+      fifo.read_lines(lines)
+      assert_equal [], lines
+      assert_equal 0, fifo.buffer.bytesize
+      assert_equal 0, fifo.reading_bytesize
     end
   end
 end
diff --git a/test/plugin/in_tail/test_io_handler.rb b/test/plugin/in_tail/test_io_handler.rb
index 9397006763..5174ff9cfb 100644
--- a/test/plugin/in_tail/test_io_handler.rb
+++ b/test/plugin/in_tail/test_io_handler.rb
@@ -148,33 +148,144 @@ def create_watcher
     end
   end
 
-  test 'does not call receive_lines when line_size exceeds max_line_size' do
-    t = 'x' * (8192)
-    text = "#{t}\n"
+  sub_test_case 'max_line_size' do
+    test 'does not call receive_lines when line_size exceeds max_line_size' do
+      t = 'x' * (8192)
+      text = "#{t}\n"
 
-    max_line_size = 8192
+      max_line_size = 8192
 
-    @file.write(text)
-    @file.close
+      @file.write(text)
+      @file.close
 
-    update_pos = 0
+      update_pos = 0
 
-    watcher = create_watcher
-    stub(watcher).pe do
-      pe = 'position_file'
-      stub(pe).read_pos {0}
-      stub(pe).update_pos { |val| update_pos = val }
-      pe
+      watcher = create_watcher
+      stub(watcher).pe do
+        pe = 'position_file'
+        stub(pe).read_pos {0}
+        stub(pe).update_pos { |val| update_pos = val }
+        pe
+      end
+
+      returned_lines = []
+      r = Fluent::Plugin::TailInput::TailWatcher::IOHandler.new(watcher, path: @file.path, read_lines_limit: 1000, read_bytes_limit_per_second: -1, max_line_size: max_line_size, log: $log, open_on_every_update: false, metrics: @metrics) do |lines, _watcher|
+        returned_lines << lines.dup
+        true
+      end
+
+      r.on_notify
+      assert_equal text.bytesize, update_pos
+      assert_equal 0, returned_lines.size
     end
 
-    returned_lines = []
-    r = Fluent::Plugin::TailInput::TailWatcher::IOHandler.new(watcher, path: @file.path, read_lines_limit: 1000, read_bytes_limit_per_second: -1, max_line_size: max_line_size, log: $log, open_on_every_update: false, metrics: @metrics) do |lines, _watcher|
-      returned_lines << lines.dup
-      true
+    data(
+      "open_on_every_update false" => false,
+      "open_on_every_update true" => true,
+    )
+    test 'manage pos correctly if a long line not having EOL occurs' do |open_on_every_update|
+      max_line_size = 20
+      returned_lines = []
+      pos = 0
+
+      watcher = create_watcher
+      stub(watcher).pe do
+        pe = 'position_file'
+        stub(pe).read_pos { pos }
+        stub(pe).update_pos { |val| pos = val }
+        pe
+      end
+
+      io_handler = Fluent::Plugin::TailInput::TailWatcher::IOHandler.new(
+        watcher, path: @file.path, read_lines_limit: 1000, read_bytes_limit_per_second: -1,
+        max_line_size: max_line_size, log: $log, open_on_every_update: open_on_every_update,
+        metrics: @metrics
+      ) do |lines, _watcher|
+        returned_lines << lines.dup
+        true
+      end
+
+      short_line = "short line\n"
+      long_lines = [
+        "long line still not having EOL",
+        " end of the line\n",
+      ]
+
+      @file.write(short_line)
+      @file.write(long_lines[0])
+      @file.flush
+      io_handler.on_notify
+
+      assert_equal [[short_line]], returned_lines
+      assert_equal short_line.bytesize, pos
+
+      @file.write(long_lines[1])
+      @file.flush
+      io_handler.on_notify
+
+      assert_equal [[short_line]], returned_lines
+      expected_size = short_line.bytesize + long_lines[0..1].map{|l| l.bytesize}.sum
+      assert_equal expected_size, pos
+
+      io_handler.close
     end
 
-    r.on_notify
-    assert_equal text.bytesize, update_pos
-    assert_equal 0, returned_lines.size
+    data(
+      "open_on_every_update false" => false,
+      "open_on_every_update true" => true,
+    )
+    test 'discards a subsequent data in a long line even if restarting occurs between' do |open_on_every_update|
+      max_line_size = 20
+      returned_lines = []
+      pos = 0
+
+      watcher = create_watcher
+      stub(watcher).pe do
+        pe = 'position_file'
+        stub(pe).read_pos { pos }
+        stub(pe).update_pos { |val| pos = val }
+        pe
+      end
+
+      io_handler = Fluent::Plugin::TailInput::TailWatcher::IOHandler.new(
+        watcher, path: @file.path, read_lines_limit: 1000, read_bytes_limit_per_second: -1,
+        max_line_size: max_line_size, log: $log, open_on_every_update: open_on_every_update,
+        metrics: @metrics
+      ) do |lines, _watcher|
+        returned_lines << lines.dup
+        true
+      end
+
+      short_line = "short line\n"
+      long_lines = [
+        "long line still not having EOL",
+        " end of the line\n",
+      ]
+
+      @file.write(short_line)
+      @file.write(long_lines[0])
+      @file.flush
+      io_handler.on_notify
+
+      assert_equal [[short_line]], returned_lines
+
+      io_handler.close
+      io_handler = Fluent::Plugin::TailInput::TailWatcher::IOHandler.new(
+        watcher, path: @file.path, read_lines_limit: 1000, read_bytes_limit_per_second: -1,
+        max_line_size: max_line_size, log: $log, open_on_every_update: open_on_every_update,
+        metrics: @metrics
+      ) do |lines, _watcher|
+        returned_lines << lines.dup
+        true
+      end
+
+      @file.write(long_lines[1])
+      @file.flush
+      io_handler.on_notify
+
+      assert_equal [[short_line]], returned_lines
+
+      io_handler.close
+    end
   end
 end