diff --git a/lib/fluent/plugin/formatter.rb b/lib/fluent/plugin/formatter.rb
index 9fc874d648..b524f40ecd 100644
--- a/lib/fluent/plugin/formatter.rb
+++ b/lib/fluent/plugin/formatter.rb
@@ -63,9 +63,9 @@ def configure(conf)
           super
           @newline = case newline
                      when :lf
-                       "\n"
+                       "\n".freeze
                      when :crlf
-                       "\r\n"
+                       "\r\n".freeze
                      end
         end
       end
diff --git a/lib/fluent/plugin/formatter_csv.rb b/lib/fluent/plugin/formatter_csv.rb
index 100bcbff6b..74760a1df7 100644
--- a/lib/fluent/plugin/formatter_csv.rb
+++ b/lib/fluent/plugin/formatter_csv.rb
@@ -27,7 +27,7 @@ class CsvFormatter < Formatter
       helpers :record_accessor
 
       config_param :delimiter, default: ',' do |val|
-        ['\t', 'TAB'].include?(val) ? "\t" : val
+        ['\t', 'TAB'].include?(val) ? "\t".freeze : val.freeze
       end
       config_param :force_quotes, :bool, default: true
       # "array" looks good for type of :fields, but this implementation removes tailing comma
diff --git a/lib/fluent/plugin/formatter_hash.rb b/lib/fluent/plugin/formatter_hash.rb
index d91e78995f..9b66ca15af 100644
--- a/lib/fluent/plugin/formatter_hash.rb
+++ b/lib/fluent/plugin/formatter_hash.rb
@@ -27,7 +27,7 @@ class HashFormatter < Formatter
 
       def format(tag, time, record)
         line = record.to_s
-        line << @newline.freeze if @add_newline
+        line << @newline if @add_newline
         line
       end
     end
diff --git a/lib/fluent/plugin/formatter_ltsv.rb b/lib/fluent/plugin/formatter_ltsv.rb
index eb87f98e0d..cbcab8b5e6 100644
--- a/lib/fluent/plugin/formatter_ltsv.rb
+++ b/lib/fluent/plugin/formatter_ltsv.rb
@@ -25,8 +25,8 @@ class LabeledTSVFormatter < Formatter
 
       # http://ltsv.org/
 
-      config_param :delimiter, :string, default: "\t"
-      config_param :label_delimiter, :string, default: ":"
+      config_param :delimiter, :string, default: "\t".freeze
+      config_param :label_delimiter, :string, default: ":".freeze
       config_param :add_newline, :bool, default: true
 
       # TODO: escaping for \t in values
@@ -36,7 +36,7 @@ def format(tag, time, record)
           formatted << @delimiter if formatted.length.nonzero?
           formatted << "#{label}#{@label_delimiter}#{value}"
         end
-        formatted << @newline.freeze if @add_newline
+        formatted << @newline if @add_newline
         formatted
       end
     end
diff --git a/lib/fluent/plugin/formatter_out_file.rb b/lib/fluent/plugin/formatter_out_file.rb
index 98c4126ce2..364303000a 100644
--- a/lib/fluent/plugin/formatter_out_file.rb
+++ b/lib/fluent/plugin/formatter_out_file.rb
@@ -29,9 +29,9 @@ class OutFileFormatter < Formatter
       config_param :output_tag, :bool, default: true
       config_param :delimiter, default: "\t" do |val|
         case val
-        when /SPACE/i then ' '
-        when /COMMA/i then ','
-        else "\t"
+        when /SPACE/i then ' '.freeze
+        when /COMMA/i then ','.freeze
+        else "\t".freeze
         end
       end
       config_set_default :time_type, :string
diff --git a/lib/fluent/plugin/formatter_single_value.rb b/lib/fluent/plugin/formatter_single_value.rb
index 569ded8479..ed28236da8 100644
--- a/lib/fluent/plugin/formatter_single_value.rb
+++ b/lib/fluent/plugin/formatter_single_value.rb
@@ -23,12 +23,12 @@ class SingleValueFormatter < Formatter
 
       Plugin.register_formatter('single_value', self)
 
-      config_param :message_key, :string, default: 'message'
+      config_param :message_key, :string, default: 'message'.freeze
       config_param :add_newline, :bool, default: true
 
       def format(tag, time, record)
         text = record[@message_key].to_s.dup
-        text << @newline.freeze if @add_newline
+        text << @newline if @add_newline
         text
       end
     end
diff --git a/lib/fluent/plugin/formatter_tsv.rb b/lib/fluent/plugin/formatter_tsv.rb
index df14c662cd..5b4df2dfe7 100644
--- a/lib/fluent/plugin/formatter_tsv.rb
+++ b/lib/fluent/plugin/formatter_tsv.rb
@@ -26,13 +26,13 @@ class TSVFormatter < Formatter
       desc 'Field names included in each lines'
       config_param :keys, :array, value_type: :string
       desc 'The delimiter character (or string) of TSV values'
-      config_param :delimiter, :string, default: "\t"
+      config_param :delimiter, :string, default: "\t".freeze
       desc 'The parameter to enable writing to new lines'
       config_param :add_newline, :bool, default: true
 
       def format(tag, time, record)
         formatted = @keys.map{|k| record[k].to_s }.join(@delimiter)
-        formatted << @newline.freeze if @add_newline
+        formatted << @newline if @add_newline
         formatted
       end
     end