diff --git a/Gemfile.lock b/Gemfile.lock
index 44f2390..99d2268 100644
--- a/Gemfile.lock
+++ b/Gemfile.lock
@@ -3,7 +3,6 @@ PATH
   specs:
     diver_down (0.0.1.alpha1)
       activesupport (>= 7.0.0)
-      msgpack (>= 1.7.0)
       rack-contrib (>= 2.3.0)
 
 GEM
@@ -32,7 +31,6 @@ GEM
     json (2.7.2)
     language_server-protocol (3.17.0.3)
     minitest (5.22.3)
-    msgpack (1.7.2)
     mutex_m (0.2.0)
     nio4r (2.7.1)
     parallel (1.24.0)
diff --git a/README.md b/README.md
index eb3938b..6b893a8 100644
--- a/README.md
+++ b/README.md
@@ -58,7 +58,7 @@ end
 ```
 
 The analysis results should be output to a specific directory.
-Files saved in `.msgpack`, `.json`, or `.yaml` can be read by `DiverDown::Web`.
+Files saved in `.json` or `.yaml` can be read by `DiverDown::Web`.
 
 ```ruby
 dir = 'tmp/diver_down'
@@ -67,7 +67,6 @@ definition = tracer.trace do
   # do something
 end
 
-File.binwrite(File.join(dir, "#{definition.title}.msgpack"), definition.to_msgpack)
 File.write(File.join(dir, "#{definition.title}.json"), definition.to_h.to_json)
 File.write(File.join(dir, "#{definition.title}.yaml"), definition.to_h.to_yaml)
 ```
diff --git a/diver_down.gemspec b/diver_down.gemspec
index e87cbb9..b466fe8 100644
--- a/diver_down.gemspec
+++ b/diver_down.gemspec
@@ -26,6 +26,5 @@ Gem::Specification.new do |spec|
   spec.require_paths = ['lib']
 
   spec.add_dependency 'activesupport', '>= 7.0.0'
-  spec.add_dependency 'msgpack', '>= 1.7.0'
   spec.add_dependency 'rack-contrib', '>= 2.3.0'
 end
diff --git a/lib/diver_down.rb b/lib/diver_down.rb
index 8af45cf..eb9d66e 100644
--- a/lib/diver_down.rb
+++ b/lib/diver_down.rb
@@ -1,6 +1,5 @@
 # frozen_string_literal: true
 
-require 'msgpack'
 require_relative 'diver_down/version'
 
 module DiverDown
diff --git a/lib/diver_down/definition.rb b/lib/diver_down/definition.rb
index be6ed86..0572057 100644
--- a/lib/diver_down/definition.rb
+++ b/lib/diver_down/definition.rb
@@ -79,11 +79,6 @@ def to_h
       }
     end
 
-    # @return [String]
-    def to_msgpack
-      MessagePack.pack(to_h)
-    end
-
     # @param other [Object, DiverDown::Definition::Source]
     # @return [Boolean]
     def ==(other)
diff --git a/lib/diver_down/web/definition_loader.rb b/lib/diver_down/web/definition_loader.rb
index 445da5c..8b989a0 100644
--- a/lib/diver_down/web/definition_loader.rb
+++ b/lib/diver_down/web/definition_loader.rb
@@ -8,8 +8,6 @@ def load_file(path)
         hash = case File.extname(path)
                when '.yaml', '.yml'
                  from_yaml(path)
-               when '.msgpack'
-                 from_msgpack(path)
                when '.json'
                  from_json(path)
                else
@@ -28,10 +26,6 @@ def from_json(path)
       def from_yaml(path)
         YAML.load_file(path)
       end
-
-      def from_msgpack(path)
-        MessagePack.unpack(File.binread(path))
-      end
     end
   end
 end
diff --git a/spec/diver_down/definition_spec.rb b/spec/diver_down/definition_spec.rb
index 9f3c15f..0b88475 100644
--- a/spec/diver_down/definition_spec.rb
+++ b/spec/diver_down/definition_spec.rb
@@ -159,66 +159,6 @@
       end
     end
 
-    describe '#to_msgpack' do
-      it 'converts definition to message-pack' do
-        definition = described_class.new(
-          title: 'title',
-          sources: [
-            DiverDown::Definition::Source.new(
-              source_name: 'a.rb',
-              dependencies: [
-                DiverDown::Definition::Dependency.new(
-                  source_name: 'b.rb',
-                  method_ids: [
-                    DiverDown::Definition::MethodId.new(
-                      name: 'A',
-                      context: 'class',
-                      paths: ['a.rb']
-                    ),
-                  ]
-                ),
-                DiverDown::Definition::Dependency.new(
-                  source_name: 'c.rb'
-                ),
-              ]
-            ),
-          ]
-        )
-
-        expect(definition.to_msgpack).to eq(MessagePack.pack(definition.to_h))
-      end
-
-      it 'can be loaded' do
-        definition = described_class.new(
-          title: 'title',
-          sources: [
-            DiverDown::Definition::Source.new(
-              source_name: 'a.rb',
-              dependencies: [
-                DiverDown::Definition::Dependency.new(
-                  source_name: 'b.rb',
-                  method_ids: [
-                    DiverDown::Definition::MethodId.new(
-                      name: 'A',
-                      context: 'class',
-                      paths: ['a.rb']
-                    ),
-                  ]
-                ),
-                DiverDown::Definition::Dependency.new(
-                  source_name: 'c.rb'
-                ),
-              ]
-            ),
-          ]
-        )
-
-        msgpack = definition.to_msgpack
-        hash = DiverDown::Helper.deep_symbolize_keys(MessagePack.unpack(msgpack))
-        expect(described_class.from_hash(hash)).to eq(definition)
-      end
-    end
-
     describe '#hash' do
       it 'returns a hash' do
         definition = described_class.new(
diff --git a/spec/diver_down/web/definition_loader_spec.rb b/spec/diver_down/web/definition_loader_spec.rb
index a57775d..5b836ec 100644
--- a/spec/diver_down/web/definition_loader_spec.rb
+++ b/spec/diver_down/web/definition_loader_spec.rb
@@ -38,18 +38,6 @@
 
         expect(loader.load_file(path)).to eq(definition)
       end
-
-      it 'loads msgpack' do
-        dir = Dir.mktmpdir
-        path = File.join(dir, 'a.msgpack')
-
-        definition = DiverDown::Definition.new(title: 'a')
-        File.write(path, definition.to_h.to_msgpack)
-
-        loader = DiverDown::Web::DefinitionLoader.new
-
-        expect(loader.load_file(path)).to eq(definition)
-      end
     end
   end
 end
diff --git a/spec/diver_down/web_spec.rb b/spec/diver_down/web_spec.rb
index 733a2b0..bc15d49 100644
--- a/spec/diver_down/web_spec.rb
+++ b/spec/diver_down/web_spec.rb
@@ -206,7 +206,7 @@ def assert_source(source, expected_ids)
           ),
         ]
       )
-      File.binwrite(File.join(definition_dir, '1.msgpack'), definition.to_h.to_msgpack)
+      File.write(File.join(definition_dir, '1.yaml'), definition.to_h.to_yaml)
 
       get '/api/initialization_status.json'