diff --git a/lib/fluent/plugin/in_tail.rb b/lib/fluent/plugin/in_tail.rb
index 3cd9363b3e..9eda2ab47d 100644
--- a/lib/fluent/plugin/in_tail.rb
+++ b/lib/fluent/plugin/in_tail.rb
@@ -82,7 +82,7 @@ def initialize
     desc 'The number of reading lines at each IO.'
     config_param :read_lines_limit, :integer, default: 1000
     desc 'The number of reading bytes per second'
-    config_param :read_bytes_limit_per_second, :integer, default: -1
+    config_param :read_bytes_limit_per_second, :size, default: -1
     desc 'The interval of flushing the buffer for multiline format'
     config_param :multiline_flush_interval, :time, default: nil
     desc 'Enable the option to emit unmatched lines.'
diff --git a/test/plugin/test_in_tail.rb b/test/plugin/test_in_tail.rb
index a2d62c6396..105ef74a2f 100644
--- a/test/plugin/test_in_tail.rb
+++ b/test/plugin/test_in_tail.rb
@@ -252,10 +252,14 @@ def test_emit_with_read_lines_limit(data)
         FileUtils.rm_f("#{TMP_DIR}/tail.txt")
       end
 
-      data("flat 8192 bytes, 10 events" => [:flat, 100, 8192, 10],
-           "flat #{8192*10} bytes, 20 events" => [:flat, 100, (8192 * 10), 20],
-           "parse #{8192*3} bytes, 10 events" => [:parse, 100, (8192 * 3), 10],
-           "parse #{8192*10} bytes, 20 events" => [:parse, 100, (8192 * 10), 20])
+      data("flat 8192 bytes, 10 events"        => [:flat, 100, 8192, 10],
+           "flat #{8192*10} bytes, 20 events"  => [:flat, 100, (8192 * 10), 20],
+           "parse #{8192*3} bytes, 10 events"  => [:parse, 100, (8192 * 3), 10],
+           "parse #{8192*10} bytes, 20 events" => [:parse, 100, (8192 * 10), 20],
+           "flat 8k bytes with unit, 10 events"        => [:flat, 100, "8k", 10],
+           "flat #{8*10}k bytes with unit, 20 events"  => [:flat, 100, "#{8*10}k", 20],
+           "parse #{8*3}k bytes with unit, 10 events"  => [:parse, 100, "#{8*3}k", 10],
+           "parse #{8*10}k bytes with unit, 20 events" => [:parse, 100, "#{8*10}k", 20])
       def test_emit_with_read_bytes_limit_per_second(data)
         config_style, limit, limit_bytes, num_events = data
         case config_style