diff --git a/.travis.yml b/.travis.yml
index 015168e..ccc4a3c 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -3,8 +3,8 @@ sudo: false
 language: ruby
 cache: bundler
 rvm:
-  - 2.6.3
+  - 3.2.1
 before_install:
-  - gem install bundler -v 2.0.2
+  - gem install bundler -v 2.3.7
 script:
   - bundle exec rspec
diff --git a/Gemfile.lock b/Gemfile.lock
index 123e550..704b077 100644
--- a/Gemfile.lock
+++ b/Gemfile.lock
@@ -7,27 +7,27 @@ PATH
 GEM
   remote: https://rubygems.org/
   specs:
-    coderay (1.1.2)
-    diff-lcs (1.3)
-    google-protobuf (3.11.4)
-    method_source (0.9.2)
-    pry (0.12.2)
-      coderay (~> 1.1.0)
-      method_source (~> 0.9.0)
-    rake (13.0.1)
-    rspec (3.9.0)
-      rspec-core (~> 3.9.0)
-      rspec-expectations (~> 3.9.0)
-      rspec-mocks (~> 3.9.0)
-    rspec-core (3.9.1)
-      rspec-support (~> 3.9.1)
-    rspec-expectations (3.9.0)
+    coderay (1.1.3)
+    diff-lcs (1.5.0)
+    google-protobuf (3.25.1)
+    method_source (1.0.0)
+    pry (0.14.2)
+      coderay (~> 1.1)
+      method_source (~> 1.0)
+    rake (13.1.0)
+    rspec (3.12.0)
+      rspec-core (~> 3.12.0)
+      rspec-expectations (~> 3.12.0)
+      rspec-mocks (~> 3.12.0)
+    rspec-core (3.12.2)
+      rspec-support (~> 3.12.0)
+    rspec-expectations (3.12.3)
       diff-lcs (>= 1.2.0, < 2.0)
-      rspec-support (~> 3.9.0)
-    rspec-mocks (3.9.1)
+      rspec-support (~> 3.12.0)
+    rspec-mocks (3.12.6)
       diff-lcs (>= 1.2.0, < 2.0)
-      rspec-support (~> 3.9.0)
-    rspec-support (3.9.2)
+      rspec-support (~> 3.12.0)
+    rspec-support (3.12.1)
 
 PLATFORMS
   ruby
diff --git a/lib/pb.rb b/lib/pb.rb
index 62f08fc..5d47483 100644
--- a/lib/pb.rb
+++ b/lib/pb.rb
@@ -212,13 +212,34 @@ def to_proto_one(klass, v)
         type_info = field_types[k.to_s]
         next if type_info.nil?  # Ignore unknown field
 
-        if type_info.type == :message
-          params[k] = to_proto(type_info.subtype.msgclass, vv)
+        params[k] = serialize(type_info, vv)
+      end
+
+      klass.new(params)
+    end
+
+    # klass is special _MapEntry_ message class
+    def to_proto_map(klass, v)
+      key_descriptor = klass.descriptor.entries.find { |e| e.name == 'key' }
+      value_descriptor = klass.descriptor.entries.find { |e| e.name == 'value' }
+      v.to_h do |k, vv|
+        [
+          serialize(key_descriptor, k),
+          serialize(value_descriptor, vv)
+        ]
+      end
+    end
+
+    def serialize(descriptor, value)
+      if descriptor.type == :message
+        if descriptor.subtype.options.map_entry
+          to_proto_map(descriptor.subtype.msgclass, value)
         else
-          params[k] = to_primitive(type_info.type, vv)
+          to_proto(descriptor.subtype.msgclass, value)
         end
+      else
+        to_primitive(descriptor.type, value)
       end
-      klass.new(params)
     end
 
     # @param [Symbol] type The type of protobuf field. e.g. :enum, :int64,
diff --git a/spec/pb_spec.rb b/spec/pb_spec.rb
index 75f7121..0d923cc 100644
--- a/spec/pb_spec.rb
+++ b/spec/pb_spec.rb
@@ -300,6 +300,67 @@
           )
         )
       end
+
+    end
+    context "when message entry is a map" do
+      let(:proto_class_a) {
+        klass_b = proto_class_b  # Assign to variable
+
+        # Use random name to avoid conflict
+        proto_class_name = "proto_class_a_#{Array.new(20) { rand(('a'.ord)..('z'.ord)).chr }.join}"
+
+        Google::Protobuf::DescriptorPool.generated_pool.build do
+          add_message proto_class_name do
+            map :map_with_primitive_value, :string, :string, 1
+            map :map_with_message_value, :string, :message, 2, klass_b.descriptor.name
+            map :map_with_builtin_value, :string, :message, 3, "google.protobuf.Timestamp"
+          end
+        end
+
+        Google::Protobuf::DescriptorPool.generated_pool.lookup(proto_class_name).msgclass
+      }
+      let(:proto_class_b) {
+        # Use random name to avoid conflict
+        proto_class_name = "proto_class_b_#{Array.new(20) { rand(('a'.ord)..('z'.ord)).chr }.join}"
+
+        Google::Protobuf::DescriptorPool.generated_pool.build do
+          add_message proto_class_name do
+            optional :id, :int64, 1
+          end
+        end
+
+        Google::Protobuf::DescriptorPool.generated_pool.lookup(proto_class_name).msgclass
+      }
+
+      it "returns proto object" do
+        expect(Pb.to_proto(proto_class_a, {})).to eq proto_class_a.new
+
+        expect(Pb.to_proto(proto_class_a, {
+          map_with_primitive_value: {
+            "foo": "bar",
+          },
+          map_with_message_value: {
+            "foo": {
+              "id": 123
+            },
+          },
+          map_with_builtin_value: {
+            "foo": "2019-02-03T00:00:00+09:00",
+          }
+        })).to eq proto_class_a.new({
+          map_with_primitive_value: {
+            "foo": "bar",
+          },
+          map_with_message_value: {
+            "foo": {
+              "id": 123
+            },
+          },
+          map_with_builtin_value: {
+            "foo": Google::Protobuf::Timestamp.new(seconds: Time.new(2019, 2, 3).to_i),
+          }
+        })
+      end
     end
   end
 end