diff --git a/lib/fluent/command/binlog_reader.rb b/lib/fluent/command/binlog_reader.rb
index f77c47c4ff..ba7adf430e 100644
--- a/lib/fluent/command/binlog_reader.rb
+++ b/lib/fluent/command/binlog_reader.rb
@@ -130,7 +130,7 @@ def initialize(argv = ARGV)
     private
 
     def configure_option_parser
-      @options.merge!(config_params: {})
+      @options[:config_params] = {}
 
       @opt_parser.banner = "Usage: fluent-binlog-reader #{self.class.to_s.split('::').last.downcase} [options] file"
 
diff --git a/lib/fluent/config/configure_proxy.rb b/lib/fluent/config/configure_proxy.rb
index 4db4fd4ef0..ec2ce42040 100644
--- a/lib/fluent/config/configure_proxy.rb
+++ b/lib/fluent/config/configure_proxy.rb
@@ -201,7 +201,7 @@ def merge_for_finalized(other)
 
       def overwrite_defaults(other) # other is owner plugin's corresponding proxy
         self.defaults = self.defaults.merge(other.defaults)
-        self.sections.keys.each do |section_key|
+        self.sections.each_key do |section_key|
           if other.sections.has_key?(section_key)
             self.sections[section_key].overwrite_defaults(other.sections[section_key])
           end
@@ -274,7 +274,7 @@ def parameter_configuration(name, type = nil, **kwargs, &block)
         option_value_type!(name, opts, :deprecated, String)
         option_value_type!(name, opts, :obsoleted, String)
         if type == :enum
-          if !opts.has_key?(:list) || !opts[:list].is_a?(Array) || opts[:list].empty? || !opts[:list].all?{|v| v.is_a?(Symbol) }
+          if !opts.has_key?(:list) || !opts[:list].is_a?(Array) || opts[:list].empty? || !opts[:list].all?(Symbol)
             raise ArgumentError, "#{name}: enum parameter requires :list of Symbols"
           end
         end
diff --git a/lib/fluent/config/types.rb b/lib/fluent/config/types.rb
index 98e7d0d9d8..9c9ba89c8d 100644
--- a/lib/fluent/config/types.rb
+++ b/lib/fluent/config/types.rb
@@ -123,7 +123,7 @@ def self.enum_value(val, opts = {}, name = nil)
 
       s = val.to_sym
       list = opts[:list]
-      raise "Plugin BUG: config type 'enum' requires :list of symbols" unless list.is_a?(Array) && list.all?{|v| v.is_a? Symbol }
+      raise "Plugin BUG: config type 'enum' requires :list of symbols" unless list.is_a?(Array) && list.all?(Symbol)
       unless list.include?(s)
         raise ConfigError, "valid options are #{list.join(',')} but got #{val}"
       end
diff --git a/lib/fluent/configurable.rb b/lib/fluent/configurable.rb
index aa3ece1752..9131738c11 100644
--- a/lib/fluent/configurable.rb
+++ b/lib/fluent/configurable.rb
@@ -31,13 +31,13 @@ def initialize
       super
       # to simulate implicit 'attr_accessor' by config_param / config_section and its value by config_set_default
       proxy = self.class.merged_configure_proxy
-      proxy.params.keys.each do |name|
+      proxy.params.each_key do |name|
         next if name.to_s.start_with?('@')
         if proxy.defaults.has_key?(name)
           instance_variable_set("@#{name}".to_sym, proxy.defaults[name])
         end
       end
-      proxy.sections.keys.each do |name|
+      proxy.sections.each_key do |name|
         next if name.to_s.start_with?('@')
         subproxy = proxy.sections[name]
         if subproxy.multi?
diff --git a/lib/fluent/counter/mutex_hash.rb b/lib/fluent/counter/mutex_hash.rb
index 9c66286458..22868fcdda 100644
--- a/lib/fluent/counter/mutex_hash.rb
+++ b/lib/fluent/counter/mutex_hash.rb
@@ -54,7 +54,7 @@ def synchronize(*keys)
               if mutex.try_lock
                 locks[key] = mutex
               else
-                locks.values.each(&:unlock)
+                locks.each_value(&:unlock)
                 locks = {}          # flush locked keys
                 break
               end
diff --git a/lib/fluent/plugin/buf_file.rb b/lib/fluent/plugin/buf_file.rb
index d75f789830..7f11478a01 100644
--- a/lib/fluent/plugin/buf_file.rb
+++ b/lib/fluent/plugin/buf_file.rb
@@ -56,7 +56,7 @@ def configure(conf)
 
         @variable_store = Fluent::VariableStore.fetch_or_build(:buf_file)
 
-        multi_workers_configured = owner.system_config.workers > 1 ? true : false
+        multi_workers_configured = owner.system_config.workers > 1
 
         using_plugin_root_dir = false
         unless @path
diff --git a/lib/fluent/plugin_helper/metrics.rb b/lib/fluent/plugin_helper/metrics.rb
index 7489732918..452610ab75 100644
--- a/lib/fluent/plugin_helper/metrics.rb
+++ b/lib/fluent/plugin_helper/metrics.rb
@@ -65,9 +65,9 @@ def metrics_create(namespace: "fluentd", subsystem: "metrics", name:, help_text:
         metrics.configure(config)
         # For multi workers environment, cmetrics should be distinguish with static labels.
         if Fluent::Engine.system_config.workers > 1
-          labels.merge!(worker_id: fluentd_worker_id.to_s)
+          labels[:worker_id] = fluentd_worker_id.to_s
         end
-        labels.merge!(plugin: @plugin_type_or_id)
+        labels[:plugin] = @plugin_type_or_id
         metrics.create(namespace: namespace, subsystem: subsystem, name: name, help_text: help_text, labels: labels)
 
         @_metrics["#{@plugin_type_or_id}_#{namespace}_#{subsystem}_#{name}"] = metrics
diff --git a/lib/fluent/test/output_test.rb b/lib/fluent/test/output_test.rb
index b7ff3fd37a..bb18884dc2 100644
--- a/lib/fluent/test/output_test.rb
+++ b/lib/fluent/test/output_test.rb
@@ -139,7 +139,7 @@ def run(&block)
             assert_equal(@expected_buffer, buffer)
           end
 
-          lines.keys.each do |meta|
+          lines.each_key do |meta|
             chunk = @instance.buffer.generate_chunk(meta).staged!
             chunk.append(lines[meta])
             begin