diff --git a/HISTORY.md b/HISTORY.md
index 8d3a36b..aa0c124 100644
--- a/HISTORY.md
+++ b/HISTORY.md
@@ -1,3 +1,6 @@
+## master
+  * Added support for Psych 4
+
 ## 3.0.7
   * 監査ログ関連で状態管理が発生するような機能追加に備えてスレッドセーフティを事前に強化
   * ffi のバージョンを 1.17.0 以下に固定
diff --git a/bizside_test_app/Gemfile b/bizside_test_app/Gemfile
index 2cdfd5c..49781f8 100644
--- a/bizside_test_app/Gemfile
+++ b/bizside_test_app/Gemfile
@@ -25,6 +25,11 @@ gem 'sqlite3', '>= 1.3', '< 1.6.0'
 gem 'turbolinks'
 gem 'uglifier', '>= 1.3.0'
 
+# Prevent error `uninitialized constant ActiveSupport::LoggerThreadSafeLevel::Logger (NameError)`
+if rails_version < '7.1'
+  gem 'concurrent-ruby', '1.3.4'
+end
+
 group :development, :test do
   gem 'byebug'
   gem 'cucumber', '~> 7.1'
diff --git a/lib/bizside.rb b/lib/bizside.rb
index f7d797b..affb064 100644
--- a/lib/bizside.rb
+++ b/lib/bizside.rb
@@ -51,11 +51,10 @@ def self.config
     end
 
     configfile = ENV['CONFIG_FILE'] || File.join('config', 'bizside.yml')
-    if File.exist?(configfile)
-      @@_config = Bizside::Config.new(YAML.load_file(configfile)[Bizside.env])
-    else
-      raise "設定ファイルの #{configfile} は必須です。"
-    end
+    raise "設定ファイルの #{configfile} は必須です。" unless File.exist?(configfile)
+
+    entire_config = YAML.respond_to?(:safe_load_file) ? YAML.safe_load_file(configfile, aliases: true) : YAML.load_file(configfile)
+    @@_config = Bizside::Config.new(entire_config[Bizside.env])
   end
 
   def self.logger
diff --git a/lib/bizside/acl/access_control_utils.rb b/lib/bizside/acl/access_control_utils.rb
index 601eb2f..04e7d9d 100755
--- a/lib/bizside/acl/access_control_utils.rb
+++ b/lib/bizside/acl/access_control_utils.rb
@@ -14,9 +14,8 @@ def self.init(reload = false)
   end
 
   def self.merge(filename)
-    yml = YAML.load_file(filename)
-    
-    yml.each do |roll_key, values|
+    entire_config = YAML.respond_to?(:safe_load_file) ? YAML.safe_load_file(filename, aliases: true) : YAML.load_file(filename)
+    entire_config.each do |roll_key, values|
       @@access_control[roll_key] ||= {}
       @@access_control[roll_key].merge!(values)
     end
@@ -33,4 +32,4 @@ def self.get_access_control(roll_key)
     @@access_control[roll_key]
   end
   
-end
\ No newline at end of file
+end
diff --git a/lib/bizside/carrierwave.rb b/lib/bizside/carrierwave.rb
index a69783f..0793a1c 100644
--- a/lib/bizside/carrierwave.rb
+++ b/lib/bizside/carrierwave.rb
@@ -27,7 +27,8 @@ def path
 
 CarrierWave.configure do |config|
   database_yml = ERB.new(File.read(File.join('config', 'database.yml')), 0, '-').result
-  database = YAML.load(database_yml)[Bizside.env]['database']
+  entire_config = YAML.respond_to?(:safe_load) ? YAML.safe_load(database_yml, aliases: true) : YAML.load(database_yml)
+  database = entire_config[Bizside.env]['database']
 
   config.root = File.join('/data', Bizside.config.add_on_name, database)
 
diff --git a/lib/bizside/configurations/mail.rb b/lib/bizside/configurations/mail.rb
index fd7b440..dc2e488 100644
--- a/lib/bizside/configurations/mail.rb
+++ b/lib/bizside/configurations/mail.rb
@@ -3,17 +3,16 @@ module Configurations
     module Mail
 
       def mail
-        if @mail.nil?
-          configfile = File.join('config', 'mail.yml')
+        return @mail if defined? @mail
+
+        configfile = File.join('config', 'mail.yml')
         
-          if File.exist?(configfile)
-            @mail = Bizside::Config.new(YAML.load_file(configfile)[Bizside.env])
+        @mail = if File.exist?(configfile)
+            entire_config = YAML.respond_to?(:safe_load_file) ? YAML.safe_load_file(configfile, aliases: true) : YAML.load_file(configfile)
+            Bizside::Config.new(entire_config[Bizside.env])
           else
-            @mail = Bizside::Config.new
+            Bizside::Config.new
           end
-        end
-
-        @mail
       end
 
       def default_url_options
diff --git a/lib/bizside/configurations/storage.rb b/lib/bizside/configurations/storage.rb
index b816f56..2c7e749 100644
--- a/lib/bizside/configurations/storage.rb
+++ b/lib/bizside/configurations/storage.rb
@@ -3,18 +3,16 @@ module Configurations
     module Storage
 
       def storage
-        if @storage.nil?
-          configfile = ENV['STORAGE_CONFIG_FILE'] ? ENV['STORAGE_CONFIG_FILE'] : default_configfile
+        return @storage if defined? @storage
 
-          if File.exist?(configfile)
-            config = ERB.new(File.read(configfile), 0, '-').result
-            @storage = Bizside::Config.new(YAML.load(config)[Bizside.env])
+        configfile = ENV['STORAGE_CONFIG_FILE'] ? ENV['STORAGE_CONFIG_FILE'] : default_configfile
+        @storage = if File.exist?(configfile)
+            text = ERB.new(File.read(configfile), 0, '-').result
+            entire_config = YAML.respond_to?(:safe_load) ? YAML.safe_load(text, aliases: true) : YAML.load(text)
+            Bizside::Config.new(entire_config[Bizside.env])
           else
-            @storage = Bizside::Config.new
+            Bizside::Config.new
           end
-        end
-
-        @storage
       end
 
       private
diff --git a/lib/bizside/gengou.rb b/lib/bizside/gengou.rb
index 0ae6c39..378fb94 100755
--- a/lib/bizside/gengou.rb
+++ b/lib/bizside/gengou.rb
@@ -2,7 +2,9 @@
 
 module Bizside
   class Gengou
-    @@_gengou = YAML.load_file(File.join(File.dirname(__FILE__), 'gengou.yml'))
+    @@_gengou = File.join(__dir__, 'gengou.yml').then do |filename|
+      YAML.respond_to?(:safe_load_file) ? YAML.safe_load_file(filename, permitted_classes: [Date]) : YAML.load_file(filename)
+    end
 
     def self.to_seireki(gengou, year_jp)
       # 引数 year_jpには年度の値が入る
diff --git a/lib/bizside/hanaita_conf.rb b/lib/bizside/hanaita_conf.rb
index f6bcda6..6cb5f71 100644
--- a/lib/bizside/hanaita_conf.rb
+++ b/lib/bizside/hanaita_conf.rb
@@ -10,7 +10,10 @@ class HanaitaConfSub
 
     def initialize
       conf_file = ENV['HANAITA_CONF'] || CONF_FILE
-      @_conf = YAML.load_file(conf_file) if File.exist?(conf_file)
+      @_conf = conf_file.then do |filename|
+        next nil unless File.exist?(filename)
+        YAML.respond_to?(:safe_load_file) ? YAML.safe_load_file(filename, aliases: true) : YAML.load_file(filename)
+      end
     end
 
     def conf
diff --git a/lib/bizside/itamae_conf.rb b/lib/bizside/itamae_conf.rb
index d026560..3a1c926 100644
--- a/lib/bizside/itamae_conf.rb
+++ b/lib/bizside/itamae_conf.rb
@@ -41,7 +41,8 @@ def initialize
       for conf_file in self.class.conf_files do
         if File.exist?(conf_file)
           @_conf ||= {}
-          hash = YAML.load(ERB.new(File.read(conf_file)).result)
+          text = ERB.new(File.read(conf_file)).result
+          hash = YAML.respond_to?(:safe_load) ? YAML.safe_load(text, aliases: true) : YAML.load(text)
 
           case conf_file
           # itamae.yml スペシャルロジック。ROLE必須。
diff --git a/lib/bizside/resque.rb b/lib/bizside/resque.rb
index 358a83d..ea398c9 100644
--- a/lib/bizside/resque.rb
+++ b/lib/bizside/resque.rb
@@ -14,19 +14,20 @@ module Resque
   yaml: ['config/resque.yml', 'config/redis.yml'],
   json: ['config/resque.json', 'config/redis.json']
 }.each do |format, file_candidates|
+  resque_config_loader = case format
+    when :yaml
+      ->(text) { YAML.respond_to?(:safe_load) ? YAML.safe_load(text, aliases: true) : YAML.load(text) }
+    when :json
+      ->(text) { ActiveSupport::JSON.decode(text) }
+    end
+
   file_candidates.each do |file|
     resque_file = File.join(File.expand_path(ENV['RAILS_ROOT'] || '.'), file)
     next unless File.exist?(resque_file)
 
-    _resque_config = ERB.new(File.read(resque_file), 0, '-').result
-  
-    case format
-    when :yaml
-      resque_config = YAML.safe_load(_resque_config)[Bizside.env]
-    when :json
-      resque_config = ActiveSupport::JSON.decode(_resque_config)[Bizside.env]
-    else
-      raise "不正なResque設定ファイルです。#{file}"
+    resque_config = ERB.new(File.read(resque_file), 0, '-').result.then do |text|
+      entire_config = resque_config_loader.call(text)
+      entire_config[Bizside.env]
     end
 
     if resque_config.is_a?(Hash)
diff --git a/lib/bizside/task_helper.rb b/lib/bizside/task_helper.rb
index aeb6fce..caf473a 100644
--- a/lib/bizside/task_helper.rb
+++ b/lib/bizside/task_helper.rb
@@ -176,7 +176,10 @@ def self.ask_yes(prompt, options = {})
 
 def self.ask_env(env_key, options = {})
   cache_file = 'tmp/cache/env'
-  cache = File.exist?(cache_file) ? YAML.load_file(cache_file) : {}
+  cache = cache_file.then do |filename|
+    next {} unless File.exist?(filename)
+    YAML.respond_to?(:unsafe_load_file) ? YAML.unsafe_load_file(filename) : YAML.load_file(filename)
+  end
 
   if options.fetch(:cache, false)
     options = options.merge(default: cache.fetch(env_key, options[:default]))
diff --git a/lib/bizside/uploader/extension_whitelist.rb b/lib/bizside/uploader/extension_whitelist.rb
index ba7e980..f99bdfe 100644
--- a/lib/bizside/uploader/extension_whitelist.rb
+++ b/lib/bizside/uploader/extension_whitelist.rb
@@ -4,9 +4,11 @@ module ExtensionWhitelist
       extend ActiveSupport::Concern
 
       included do
-        default_extensions = Bizside.config.file_uploader.extensions_file_path.present? ? Bizside.config.file_uploader.extensions_file_path : 
-          File.join(File.dirname(__FILE__), 'default_extensions.yml')
-        @@extensions = YAML.load_file(default_extensions).values
+        @@extensions = Bizside.config.file_uploader.extensions_file_path.then do |filename|
+          filename = File.join(__dir__, 'default_extensions.yml') unless filename.present?
+          entire_config = YAML.respond_to?(:safe_load_file) ? YAML.safe_load_file(filename, aliases: true) : YAML.load_file(filename)
+          entire_config.values
+        end
       end
       
       def extension_allowlist