diff --git a/lib/fluent/command/ctl.rb b/lib/fluent/command/ctl.rb
index f908d40f05..11e59c9a30 100644
--- a/lib/fluent/command/ctl.rb
+++ b/lib/fluent/command/ctl.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 #
 # Fluentd
 #
@@ -67,7 +69,7 @@ def initialize(argv = ARGV)
     end
 
     def help_text
-      text = "\n"
+      text = +"\n"
       if Fluent.windows?
         text << "Usage: #{$PROGRAM_NAME} COMMAND [PID_OR_SVCNAME]\n"
       else
diff --git a/lib/fluent/command/plugin_config_formatter.rb b/lib/fluent/command/plugin_config_formatter.rb
index 00d660da0b..7540733d8d 100644
--- a/lib/fluent/command/plugin_config_formatter.rb
+++ b/lib/fluent/command/plugin_config_formatter.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 #
 # Fluentd
 #
@@ -79,7 +81,7 @@ def call
   private
 
   def dump_txt(dumped_config)
-    dumped = ""
+    dumped = +""
     plugin_helpers = dumped_config.delete(:plugin_helpers)
     if plugin_helpers && !plugin_helpers.empty?
       dumped << "helpers: #{plugin_helpers.join(',')}\n"
@@ -101,7 +103,7 @@ def dump_txt(dumped_config)
   end
 
   def dump_section_txt(base_section, level = 0)
-    dumped = ""
+    dumped = +""
     indent = " " * level
     if base_section[:section]
       sections = []
@@ -135,10 +137,10 @@ def dump_section_txt(base_section, level = 0)
   end
 
   def dump_markdown(dumped_config)
-    dumped = ""
+    dumped = +""
     plugin_helpers = dumped_config.delete(:plugin_helpers)
     if plugin_helpers && !plugin_helpers.empty?
-      dumped = "## Plugin helpers\n\n"
+      dumped = +"## Plugin helpers\n\n"
       plugin_helpers.each do |plugin_helper|
         dumped << "* #{plugin_helper_markdown_link(plugin_helper)}\n"
       end
@@ -156,7 +158,7 @@ def dump_markdown(dumped_config)
   end
 
   def dump_section_markdown(base_section, level = 0)
-    dumped = ""
+    dumped = +""
     if base_section[:section]
       sections = []
       params = base_section
diff --git a/lib/fluent/compat/record_filter_mixin.rb b/lib/fluent/compat/record_filter_mixin.rb
index 38a554d8bb..a421f30b2c 100644
--- a/lib/fluent/compat/record_filter_mixin.rb
+++ b/lib/fluent/compat/record_filter_mixin.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 #
 # Fluentd
 #
@@ -21,7 +23,7 @@ def filter_record(tag, time, record)
       end
 
       def format_stream(tag, es)
-        out = ''
+        out = +''
         es.each {|time,record|
           tag_temp = tag.dup
           filter_record(tag_temp, time, record)
diff --git a/lib/fluent/config/element.rb b/lib/fluent/config/element.rb
index c61eb391cb..e53fb80ea7 100644
--- a/lib/fluent/config/element.rb
+++ b/lib/fluent/config/element.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 #
 # Fluentd
 #
@@ -141,7 +143,7 @@ def check_not_fetched(&block)
       def to_s(nest = 0)
         indent = "  " * nest
         nindent = "  " * (nest + 1)
-        out = ""
+        out = +""
         if @arg.nil? || @arg.empty?
           out << "#{indent}<#{@name}>\n"
         else
@@ -230,7 +232,7 @@ def dump_value(k, v, nindent)
       end
 
       def self.unescape_parameter(v)
-        result = ''
+        result = +''
         v.each_char { |c| result << LiteralParser.unescape_char(c) }
         result
       end
diff --git a/lib/fluent/config/literal_parser.rb b/lib/fluent/config/literal_parser.rb
index 5664986481..f34066dff8 100644
--- a/lib/fluent/config/literal_parser.rb
+++ b/lib/fluent/config/literal_parser.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 #
 # Fluentd
 #
@@ -228,8 +230,8 @@ def scan_json(is_array)
         # Yajl does not raise ParseError for incomplete json string, like '[1', '{"h"', '{"h":' or '{"h1":1'
         # This is the reason to use JSON module.
 
-        buffer = (is_array ? "[" : "{")
-        line_buffer = ""
+        buffer = +(is_array ? "[" : "{")
+        line_buffer = +""
 
         until result
           char = getch
@@ -252,7 +254,7 @@ def scan_json(is_array)
                 # ignore comment char
               end
               buffer << line_buffer + "\n"
-              line_buffer = ""
+              line_buffer = +""
             else
               # '#' is a char in json string
               line_buffer << char
@@ -263,7 +265,7 @@ def scan_json(is_array)
 
           if char == "\n"
             buffer << line_buffer + "\n"
-            line_buffer = ""
+            line_buffer = +""
             next
           end
 
diff --git a/lib/fluent/config/types.rb b/lib/fluent/config/types.rb
index 9c9ba89c8d..b1458ebbee 100644
--- a/lib/fluent/config/types.rb
+++ b/lib/fluent/config/types.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 #
 # Fluentd
 #
@@ -100,7 +102,7 @@ def self.string_value(val, opts = {}, name = nil)
       return nil if val.nil?
 
       v = val.to_s
-      v = v.frozen? ? v.dup : v # config_param can't assume incoming string is mutable
+      v = v.frozen? ? v.dup : +v # config_param can't assume incoming string is mutable
       v.force_encoding(Encoding::UTF_8)
     end
 
diff --git a/lib/fluent/log.rb b/lib/fluent/log.rb
index f8175fd911..69d632870a 100644
--- a/lib/fluent/log.rb
+++ b/lib/fluent/log.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 #
 # Fluentd
 #
@@ -550,7 +552,7 @@ def get_worker_id(type)
 
     def event(level, args)
       time = Time.now
-      message = @optional_header ? @optional_header.dup : ''
+      message = @optional_header ? @optional_header.dup : +''
       map = @optional_attrs ? @optional_attrs.dup : {}
       args.each {|a|
         if a.is_a?(Hash)
diff --git a/lib/fluent/match.rb b/lib/fluent/match.rb
index 7b6c076adc..79d0330f62 100644
--- a/lib/fluent/match.rb
+++ b/lib/fluent/match.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 #
 # Fluentd
 #
@@ -43,7 +45,7 @@ def initialize(pat)
       end
 
       stack = []
-      regex = ['']
+      regex = [+'']
       escape = false
       dot = false
 
@@ -105,7 +107,7 @@ def initialize(pat)
         elsif c == "{"
           # or
           stack.push []
-          regex.push ''
+          regex.push +''
 
         elsif c == "}" && !stack.empty?
           stack.last << regex.pop
diff --git a/lib/fluent/plugin/buffer/file_chunk.rb b/lib/fluent/plugin/buffer/file_chunk.rb
index 7930574a58..7373e8b68d 100644
--- a/lib/fluent/plugin/buffer/file_chunk.rb
+++ b/lib/fluent/plugin/buffer/file_chunk.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 #
 # Fluentd
 #
@@ -24,7 +26,7 @@ class Buffer
       class FileChunk < Chunk
         class FileChunkError < StandardError; end
 
-        BUFFER_HEADER = "\xc1\x00".force_encoding(Encoding::ASCII_8BIT).freeze
+        BUFFER_HEADER = (+"\xc1\x00").force_encoding(Encoding::ASCII_8BIT).freeze
 
         ### buffer path user specified : /path/to/directory/user_specified_prefix.*.log
         ### buffer chunk path          : /path/to/directory/user_specified_prefix.b513b61c9791029c2513b61c9791029c2.log
diff --git a/lib/fluent/plugin/buffer/memory_chunk.rb b/lib/fluent/plugin/buffer/memory_chunk.rb
index 556c8c8a3d..ebd4a8a633 100644
--- a/lib/fluent/plugin/buffer/memory_chunk.rb
+++ b/lib/fluent/plugin/buffer/memory_chunk.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 #
 # Fluentd
 #
@@ -22,7 +24,7 @@ class Buffer
       class MemoryChunk < Chunk
         def initialize(metadata, compress: :text)
           super
-          @chunk = ''.force_encoding(Encoding::ASCII_8BIT)
+          @chunk = (+'').force_encoding(Encoding::ASCII_8BIT)
           @chunk_bytes = 0
           @adding_bytes = 0
           @adding_size = 0
@@ -68,7 +70,7 @@ def empty?
 
         def purge
           super
-          @chunk = ''.force_encoding("ASCII-8BIT")
+          @chunk = (+'').force_encoding("ASCII-8BIT")
           @chunk_bytes = @size = @adding_bytes = @adding_size = 0
           true
         end
diff --git a/lib/fluent/plugin/compressable.rb b/lib/fluent/plugin/compressable.rb
index aa242478fe..73fcf94d21 100644
--- a/lib/fluent/plugin/compressable.rb
+++ b/lib/fluent/plugin/compressable.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 #
 # Fluentd
 #
@@ -57,7 +59,7 @@ def decompress(compressed_data = nil, output_io: nil, input_io: nil)
       def string_decompress(compressed_data)
         io = StringIO.new(compressed_data)
 
-        out = ''
+        out = +''
         loop do
           gz = Zlib::GzipReader.new(io)
           out << gz.read
diff --git a/lib/fluent/plugin/formatter_csv.rb b/lib/fluent/plugin/formatter_csv.rb
index 74760a1df7..6bc33db816 100644
--- a/lib/fluent/plugin/formatter_csv.rb
+++ b/lib/fluent/plugin/formatter_csv.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 #
 # Fluentd
 #
@@ -50,13 +52,13 @@ def configure(conf)
         end
 
         @generate_opts = {col_sep: @delimiter, force_quotes: @force_quotes, headers: @fields,
-                          row_sep: @add_newline ? :auto : "".force_encoding(Encoding::ASCII_8BIT)}
+                          row_sep: @add_newline ? :auto : (+"").force_encoding(Encoding::ASCII_8BIT)}
         # Cache CSV object per thread to avoid internal state sharing
         @cache = {}
       end
 
       def format(tag, time, record)
-        csv = (@cache[Thread.current] ||= CSV.new("".force_encoding(Encoding::ASCII_8BIT), **@generate_opts))
+        csv = (@cache[Thread.current] ||= CSV.new((+"").force_encoding(Encoding::ASCII_8BIT), **@generate_opts))
         line = (csv << record).string.dup
         # Need manual cleanup because CSV writer doesn't provide such method.
         csv.rewind
@@ -65,7 +67,7 @@ def format(tag, time, record)
       end
 
       def format_with_nested_fields(tag, time, record)
-        csv = (@cache[Thread.current] ||= CSV.new("".force_encoding(Encoding::ASCII_8BIT), **@generate_opts))
+        csv = (@cache[Thread.current] ||= CSV.new((+"").force_encoding(Encoding::ASCII_8BIT), **@generate_opts))
         values = @accessors.map { |a| a.call(record) }
         line = (csv << values).string.dup
         # Need manual cleanup because CSV writer doesn't provide such method.
diff --git a/lib/fluent/plugin/formatter_ltsv.rb b/lib/fluent/plugin/formatter_ltsv.rb
index 76100bb91b..1bf054ecdf 100644
--- a/lib/fluent/plugin/formatter_ltsv.rb
+++ b/lib/fluent/plugin/formatter_ltsv.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 #
 # Fluentd
 #
@@ -31,7 +33,7 @@ class LabeledTSVFormatter < Formatter
       config_param :add_newline, :bool, default: true
 
       def format(tag, time, record)
-        formatted = ""
+        formatted = +""
         record.each do |label, value|
           formatted << @delimiter if formatted.length.nonzero?
           formatted << "#{label}#{@label_delimiter}#{value.to_s.gsub(@delimiter, @replacement)}"
diff --git a/lib/fluent/plugin/formatter_out_file.rb b/lib/fluent/plugin/formatter_out_file.rb
index 364303000a..bb853d5150 100644
--- a/lib/fluent/plugin/formatter_out_file.rb
+++ b/lib/fluent/plugin/formatter_out_file.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 #
 # Fluentd
 #
@@ -43,7 +45,7 @@ def configure(conf)
       end
 
       def format(tag, time, record)
-        header = ''
+        header = +''
         header << "#{@timef.format(time)}#{@delimiter}" if @output_time
         header << "#{tag}#{@delimiter}" if @output_tag
         "#{header}#{Yajl.dump(record)}#{@newline}"
diff --git a/lib/fluent/plugin/in_http.rb b/lib/fluent/plugin/in_http.rb
index 6385a80a79..40683933ba 100644
--- a/lib/fluent/plugin/in_http.rb
+++ b/lib/fluent/plugin/in_http.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 #
 # Fluentd
 #
@@ -56,7 +58,7 @@ class HttpInput < Input
 
     helpers :parser, :compat_parameters, :event_loop, :server
 
-    EMPTY_GIF_IMAGE = "GIF89a\u0001\u0000\u0001\u0000\x80\xFF\u0000\xFF\xFF\xFF\u0000\u0000\u0000,\u0000\u0000\u0000\u0000\u0001\u0000\u0001\u0000\u0000\u0002\u0002D\u0001\u0000;".force_encoding("UTF-8")
+    EMPTY_GIF_IMAGE = (+"GIF89a\u0001\u0000\u0001\u0000\x80\xFF\u0000\xFF\xFF\xFF\u0000\u0000\u0000,\u0000\u0000\u0000\u0000\u0001\u0000\u0001\u0000\u0000\u0002\u0002D\u0001\u0000;").force_encoding("UTF-8")
 
     desc 'The port to listen to.'
     config_param :port, :integer, default: 9880
diff --git a/lib/fluent/plugin/in_monitor_agent.rb b/lib/fluent/plugin/in_monitor_agent.rb
index f41c1a0bce..0654f7b7d2 100644
--- a/lib/fluent/plugin/in_monitor_agent.rb
+++ b/lib/fluent/plugin/in_monitor_agent.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 #
 # Fluentd
 #
@@ -102,7 +104,7 @@ def render_json(obj, code: 200, pretty_json: nil)
 
       def render_ltsv(obj, code: 200)
         normalized = JSON.parse(obj.to_json)
-        text = ''
+        text = +''
         normalized.each do |hash|
           row = []
           hash.each do |k, v|
diff --git a/lib/fluent/plugin/in_syslog.rb b/lib/fluent/plugin/in_syslog.rb
index 28ad1c5dd9..f975b60d6c 100644
--- a/lib/fluent/plugin/in_syslog.rb
+++ b/lib/fluent/plugin/in_syslog.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 #
 # Fluentd
 #
diff --git a/lib/fluent/plugin/in_tail.rb b/lib/fluent/plugin/in_tail.rb
index 852f2723bd..8b84e54a0a 100644
--- a/lib/fluent/plugin/in_tail.rb
+++ b/lib/fluent/plugin/in_tail.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 #
 # Fluentd
 #
@@ -1010,7 +1012,7 @@ def initialize(from_encoding, encoding, log, max_line_size=nil)
           @from_encoding = from_encoding
           @encoding = encoding
           @need_enc = from_encoding != encoding
-          @buffer = ''.force_encoding(from_encoding)
+          @buffer = (+'').force_encoding(from_encoding)
           @eol = "\n".encode(from_encoding).freeze
           @max_line_size = max_line_size
           @skip_current_line = false
@@ -1032,6 +1034,7 @@ def <<(chunk)
           # quad-byte encoding to IO#readpartial as the second arguments results in an
           # assertion failure on Ruby < 2.4.0 for unknown reasons.
           orig_encoding = chunk.encoding
+          +chunk
           chunk.force_encoding(from_encoding)
           @buffer << chunk
           # Thus the encoding needs to be reverted back here
@@ -1118,7 +1121,7 @@ def initialize(watcher, path:, read_lines_limit:, read_bytes_limit_per_second:,
           @receive_lines = receive_lines
           @open_on_every_update = open_on_every_update
           @fifo = FIFO.new(from_encoding || Encoding::ASCII_8BIT, encoding || Encoding::ASCII_8BIT, log, max_line_size)
-          @iobuf = ''.force_encoding('ASCII-8BIT')
+          @iobuf = (+'').force_encoding('ASCII-8BIT')
           @lines = []
           @io = nil
           @notify_mutex = Mutex.new
diff --git a/lib/fluent/plugin_helper/server.rb b/lib/fluent/plugin_helper/server.rb
index eecee5ae7e..b64bcbc601 100644
--- a/lib/fluent/plugin_helper/server.rb
+++ b/lib/fluent/plugin_helper/server.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 #
 # Fluentd
 #
@@ -675,7 +677,7 @@ def initialize(sock, context, socket_option_setter, close_callback, log, under_p
             socket_option_setter.call(sock)
             @_handler_socket = OpenSSL::SSL::SSLSocket.new(sock, context)
             @_handler_socket.sync_close = true
-            @_handler_write_buffer = ''.force_encoding('ascii-8bit')
+            @_handler_write_buffer = (+'').force_encoding('ascii-8bit')
             @_handler_accepted = false
             super(@_handler_socket)
 
diff --git a/lib/fluent/test/output_test.rb b/lib/fluent/test/output_test.rb
index bb18884dc2..e5571096fe 100644
--- a/lib/fluent/test/output_test.rb
+++ b/lib/fluent/test/output_test.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 #
 # Fluentd
 #
@@ -116,7 +118,7 @@ def emit(record, time=EventTime.now)
       end
 
       def expect_format(str)
-        (@expected_buffer ||= '') << str
+        (@expected_buffer ||= +'') << str
       end
 
       def run(&block)
@@ -124,7 +126,7 @@ def run(&block)
         super {
           block.call if block
 
-          buffer = ''
+          buffer = +''
           lines = {}
           # v0.12 TimeSlicedOutput doesn't call #format_stream
           @entries.each do |time, record|