diff --git a/.github/workflows/linux.yml b/.github/workflows/linux.yml
index 37b6bfc..732dd11 100644
--- a/.github/workflows/linux.yml
+++ b/.github/workflows/linux.yml
@@ -14,7 +14,12 @@ jobs:
           - '3.4'
         os:
           - ubuntu-latest
-    name: Ruby ${{ matrix.ruby }} unit testing on ${{ matrix.os }}
+        gemfile:
+          - activesupport
+          - no-activesupport
+    env:
+      BUNDLE_GEMFILE: spec/gemfiles/${{ matrix.gemfile }}.gemfile
+    name: Ruby ${{ matrix.ruby }} unit testing on ${{ matrix.os }} (${{ matrix.gemfile }})
     steps:
     - uses: actions/checkout@v4
     - uses: ruby/setup-ruby@v1
diff --git a/.gitignore b/.gitignore
index 3d83501..464b6ee 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1,4 +1,5 @@
 .bundle
+spec/gemfiles/*.gemfile.lock
 Gemfile.lock
 pkg/*
 coverage/*
diff --git a/lib/fluent/logger/fluent_logger.rb b/lib/fluent/logger/fluent_logger.rb
index 2aef0a2..40cc793 100644
--- a/lib/fluent/logger/fluent_logger.rb
+++ b/lib/fluent/logger/fluent_logger.rb
@@ -49,6 +49,14 @@ def self.from_msgpack_ext(data)
       def to_json(*args)
         @sec.to_s
       end
+
+      def as_json(*args)
+        # For ActiveSupport.
+        # By default, the custom classes are represented by their own instance variables.
+        # https://github.com/rails/rails/blob/87a9fdeef81f293cd972464686a6b4470ec642a2/activesupport/lib/active_support/core_ext/object/json.rb#L63
+        # Define it in a way that results in proper data structure when expressed in JSON
+        @sec
+      end
     end
 
     class FluentLogger < LoggerBase
@@ -235,7 +243,7 @@ def to_msgpack(msg)
           res = begin
                   @packer.pack(msg).to_s
                 rescue NoMethodError
-                  JSON.parse(JSON.generate(msg)).to_msgpack
+                  JSON.parse(msg.to_json).to_msgpack
                 ensure
                   @packer.clear
                 end
diff --git a/spec/fluent_logger_spec.rb b/spec/fluent_logger_spec.rb
index aa67c8c..3707d66 100644
--- a/spec/fluent_logger_spec.rb
+++ b/spec/fluent_logger_spec.rb
@@ -8,6 +8,11 @@
 require 'fluent/logger/fluent_logger/cui'
 require 'timeout'
 
+begin
+  require 'active_support/json'
+rescue LoadError
+end
+
 describe Fluent::Logger::FluentLogger do
   let(:fluentd) {
     DummyFluentd.new
@@ -176,7 +181,12 @@
         logger.post('tag', data)
         fluentd.wait_transfer
         logger_data = fluentd.queue.last.last
-        expect(logger_data['time']).to eq '2008-09-01 10:05:00 UTC'
+        if defined?(ActiveSupport)
+          expect(logger_data['time']).to eq '2008-09-01T10:05:00.000Z'
+        else
+          expect(logger_data['time']).to eq '2008-09-01 10:05:00 UTC'
+        end
+
         expect(logger_data['proc']).to be_truthy
         expect(logger_data['object']).to be_truthy
       }
@@ -188,7 +198,11 @@
         fluentd.wait_transfer
 
         logger_data1 = fluentd.queue.first.last
-        expect(logger_data1['time']).to eq '2008-09-01 10:05:00 UTC'
+        if defined?(ActiveSupport)
+          expect(logger_data1['time']).to eq '2008-09-01T10:05:00.000Z'
+        else
+          expect(logger_data1['time']).to eq '2008-09-01 10:05:00 UTC'
+        end
 
         logger_data2 = fluentd.queue.last.last
         expect(logger_data2['time']).to eq '2008-09-01 10:05:00 UTC'
@@ -199,13 +213,19 @@
           'time'   => Time.utc(2008, 9, 1, 10, 5, 0),
           'object' => Object.new,
           'proc'   => proc { 1 },
-          'NaN'    => (0.0/0.0) # JSON don't convert
+          'NaN'    => (0.0/0.0) # JSON don't convert by default
         }
         logger.post('tag', data)
         fluentd.wait_transfer
-        expect(fluentd.queue.last).to be_nil
-        logger_io.rewind
-        logger_io.read =~ /FluentLogger: Can't convert to msgpack:/
+        if defined?(ActiveSupport)
+          # activesupport converts NaN to nil
+          # https://github.com/rails/rails/blob/90a1eaa1b30ba1f2d524e197460e549c03cf5698/activesupport/lib/active_support/core_ext/object/json.rb#L117-L118
+          expect(fluentd.queue.last).to eq ['logger-test.tag', {"NaN" => nil, "object" => {}, "proc" => {}, "time" => "2008-09-01T10:05:00.000Z"}]
+        else
+          expect(fluentd.queue.last).to be_nil
+          logger_io.rewind
+          expect(logger_io.read).to match(/FluentLogger: Can't convert to msgpack:/)
+        end
 
         logger.post('tag', { 'a' => 'b' })
         fluentd.wait_transfer
diff --git a/spec/gemfiles/activesupport.gemfile b/spec/gemfiles/activesupport.gemfile
new file mode 100644
index 0000000..7eff369
--- /dev/null
+++ b/spec/gemfiles/activesupport.gemfile
@@ -0,0 +1,7 @@
+# frozen_string_literal: true
+
+# Load common dependencies
+eval_gemfile("../../Gemfile")
+
+# Add custom dependencies
+gem "activesupport"
diff --git a/spec/gemfiles/no-activesupport.gemfile b/spec/gemfiles/no-activesupport.gemfile
new file mode 100644
index 0000000..4dbba7b
--- /dev/null
+++ b/spec/gemfiles/no-activesupport.gemfile
@@ -0,0 +1,4 @@
+# frozen_string_literal: true
+
+# Load common dependencies
+eval_gemfile("../../Gemfile")