diff --git a/test/command/test_fluentd.rb b/test/command/test_fluentd.rb
index 87c9d28d96..c6bd88d857 100644
--- a/test/command/test_fluentd.rb
+++ b/test/command/test_fluentd.rb
@@ -941,7 +941,7 @@ def multi_workers_ready?
       '-external-encoding' => '--external-encoding=utf-8',
       '-internal-encoding' => '--internal-encoding=utf-8',
     )
-    test "-E option is set to RUBYOPT" do |opt|
+    test "-E option is set to RUBYOPT" do |base_opt|
       conf = <<CONF
 <source>
   @type dummy
@@ -952,6 +952,7 @@ def multi_workers_ready?
 </match>
 CONF
       conf_path = create_conf_file('rubyopt_test.conf', conf)
+      opt = base_opt.dup
       opt << " #{ENV['RUBYOPT']}" if ENV['RUBYOPT']
       assert_log_matches(
         create_cmdline(conf_path),
@@ -991,9 +992,14 @@ def multi_workers_ready?
 </match>
 CONF
       conf_path = create_conf_file('rubyopt_invalid_test.conf', conf)
+      if Gem::Version.create(RUBY_VERSION) >= Gem::Version.create('3.3.0')
+        expected_phrase = 'ruby: invalid switch in RUBYOPT'
+      else
+        expected_phrase = 'Invalid option is passed to RUBYOPT'
+      end
       assert_log_matches(
         create_cmdline(conf_path),
-        'Invalid option is passed to RUBYOPT',
+        expected_phrase,
         env: { 'RUBYOPT' => 'a' },
       )
     end
diff --git a/test/plugin/test_out_forward.rb b/test/plugin/test_out_forward.rb
index 80557c5989..3a7f9a2e10 100644
--- a/test/plugin/test_out_forward.rb
+++ b/test/plugin/test_out_forward.rb
@@ -156,7 +156,14 @@ def try_write(chunk)
     normal_conf = config_element('match', '**', {}, [
         config_element('server', '', {'name' => 'test', 'host' => 'unexisting.yaaaaaaaaaaaaaay.host.example.com'})
       ])
-    assert_raise SocketError do
+
+    if Socket.const_defined?(:ResolutionError) # as of Ruby 3.3
+      error_class = Socket::ResolutionError
+    else
+      error_class = SocketError
+    end
+
+    assert_raise error_class do
       create_driver(normal_conf)
     end
 
@@ -165,7 +172,7 @@ def try_write(chunk)
       ])
     @d = d = create_driver(conf)
     expected_log = "failed to resolve node name when configured"
-    expected_detail = 'server="test" error_class=SocketError'
+    expected_detail = "server=\"test\" error_class=#{error_class.name}"
     logs = d.logs
     assert{ logs.any?{|log| log.include?(expected_log) && log.include?(expected_detail) } }
   end
diff --git a/test/plugin_helper/test_child_process.rb b/test/plugin_helper/test_child_process.rb
index 5d84a0fd06..b498b7a0eb 100644
--- a/test/plugin_helper/test_child_process.rb
+++ b/test/plugin_helper/test_child_process.rb
@@ -515,6 +515,9 @@ def configure(conf)
   end
 
   test 'can scrub characters without exceptions' do
+    if Gem::Version.create(RUBY_VERSION) >= Gem::Version.create('3.3.0')
+      pend "Behaviour of IO#set_encoding is changed as of Ruby 3.3 (#4058)"
+    end
     m = Mutex.new
     str = nil
     Timeout.timeout(TEST_DEADLOCK_TIMEOUT) do
@@ -529,19 +532,25 @@ def configure(conf)
       sleep TEST_WAIT_INTERVAL_FOR_BLOCK_RUNNING until m.locked? || ran
       m.lock
       assert_equal Encoding.find('utf-8'), str.encoding
-      expected = "\xEF\xBF\xBD\xEF\xBF\xBD\x00\xEF\xBF\xBD\xEF\xBF\xBD".force_encoding("utf-8")
+      replacement = "\uFFFD" # U+FFFD (REPLACEMENT CHARACTER)
+      nul = "\x00" # U+0000 (NUL)
+      expected = replacement * 2 + nul + replacement * 2
       assert_equal expected, str
       @d.stop; @d.shutdown; @d.close; @d.terminate
     end
   end
 
   test 'can scrub characters without exceptions and replace specified chars' do
+    if Gem::Version.create(RUBY_VERSION) >= Gem::Version.create('3.3.0')
+      pend "Behaviour of IO#set_encoding is changed as of Ruby 3.3 (#4058)"
+    end
     m = Mutex.new
     str = nil
+    replacement = "?"
     Timeout.timeout(TEST_DEADLOCK_TIMEOUT) do
       ran = false
       args = ['-e', 'STDOUT.set_encoding("ascii-8bit"); STDOUT.write "\xFF\xFF\x00\xF0\xF0"']
-      @d.child_process_execute(:t13b, "ruby", arguments: args, mode: [:read], scrub: true, replace_string: '?') do |io|
+      @d.child_process_execute(:t13b, "ruby", arguments: args, mode: [:read], scrub: true, replace_string: replacement) do |io|
         m.lock
         ran = true
         str = io.read
@@ -550,7 +559,8 @@ def configure(conf)
       sleep TEST_WAIT_INTERVAL_FOR_BLOCK_RUNNING until m.locked? || ran
       m.lock
       assert_equal Encoding.find('utf-8'), str.encoding
-      expected = "??\x00??".force_encoding("utf-8")
+      nul = "\x00" # U+0000 (NUL)
+      expected = replacement * 2 + nul + replacement * 2
       assert_equal expected, str
       @d.stop; @d.shutdown; @d.close; @d.terminate
     end