diff --git a/lib/onnxruntime/ffi.rb b/lib/onnxruntime/ffi.rb
index 1763029..8755959 100644
--- a/lib/onnxruntime/ffi.rb
+++ b/lib/onnxruntime/ffi.rb
@@ -11,7 +11,7 @@ module FFI
 
     # enums
     TensorElementDataType = enum(:undefined, :float, :uint8, :int8, :uint16, :int16, :int32, :int64, :string, :bool, :float16, :double, :uint32, :uint64, :complex64, :complex128, :bfloat16)
-    OnnxType = enum(:unknown, :tensor, :sequence, :map, :opaque, :sparsetensor)
+    OnnxType = enum(:unknown, :tensor, :sequence, :map, :opaque, :sparsetensor, :optional)
 
     class Api < ::FFI::Struct
       layout \
diff --git a/lib/onnxruntime/inference_session.rb b/lib/onnxruntime/inference_session.rb
index 88eec67..54a318e 100644
--- a/lib/onnxruntime/inference_session.rb
+++ b/lib/onnxruntime/inference_session.rb
@@ -250,7 +250,7 @@ def load_inputs
         # freed in node_info
         typeinfo = ::FFI::MemoryPointer.new(:pointer)
         check_status api[:SessionGetInputTypeInfo].call(read_pointer, i, typeinfo)
-        inputs << {name: name_ptr.read_pointer.read_string}.merge(node_info(typeinfo))
+        inputs << {name: name_ptr.read_pointer.read_string}.merge(Utils.node_info(typeinfo))
         allocator_free name_ptr
       end
       inputs
@@ -266,7 +266,7 @@ def load_outputs
         # freed in node_info
         typeinfo = ::FFI::MemoryPointer.new(:pointer)
         check_status api[:SessionGetOutputTypeInfo].call(read_pointer, i, typeinfo)
-        outputs << {name: name_ptr.read_pointer.read_string}.merge(node_info(typeinfo))
+        outputs << {name: name_ptr.read_pointer.read_string}.merge(Utils.node_info(typeinfo))
         allocator_free name_ptr
       end
       outputs
@@ -482,58 +482,6 @@ def check_status(status)
       Utils.check_status(status)
     end
 
-    def node_info(typeinfo)
-      onnx_type = ::FFI::MemoryPointer.new(:int)
-      check_status api[:GetOnnxTypeFromTypeInfo].call(typeinfo.read_pointer, onnx_type)
-
-      type = FFI::OnnxType[onnx_type.read_int]
-      case type
-      when :tensor
-        tensor_info = ::FFI::MemoryPointer.new(:pointer)
-        # don't free tensor_info
-        check_status api[:CastTypeInfoToTensorInfo].call(typeinfo.read_pointer, tensor_info)
-
-        type, shape = Utils.tensor_type_and_shape(tensor_info)
-        {
-          type: "tensor(#{FFI::TensorElementDataType[type]})",
-          shape: shape
-        }
-      when :sequence
-        sequence_type_info = ::FFI::MemoryPointer.new(:pointer)
-        check_status api[:CastTypeInfoToSequenceTypeInfo].call(typeinfo.read_pointer, sequence_type_info)
-        nested_type_info = ::FFI::MemoryPointer.new(:pointer)
-        check_status api[:GetSequenceElementType].call(sequence_type_info.read_pointer, nested_type_info)
-        v = node_info(nested_type_info)[:type]
-
-        {
-          type: "seq(#{v})",
-          shape: []
-        }
-      when :map
-        map_type_info = ::FFI::MemoryPointer.new(:pointer)
-        check_status api[:CastTypeInfoToMapTypeInfo].call(typeinfo.read_pointer, map_type_info)
-
-        # key
-        key_type = ::FFI::MemoryPointer.new(:int)
-        check_status api[:GetMapKeyType].call(map_type_info.read_pointer, key_type)
-        k = FFI::TensorElementDataType[key_type.read_int]
-
-        # value
-        value_type_info = ::FFI::MemoryPointer.new(:pointer)
-        check_status api[:GetMapValueType].call(map_type_info.read_pointer, value_type_info)
-        v = node_info(value_type_info)[:type]
-
-        {
-          type: "map(#{k},#{v})",
-          shape: []
-        }
-      else
-        Utils.unsupported_type("ONNX", type)
-      end
-    ensure
-      release :TypeInfo, typeinfo
-    end
-
     def tensor_types
       @tensor_types ||= [:float, :uint8, :int8, :uint16, :int16, :int32, :int64, :bool, :double, :uint32, :uint64].map { |v| ["tensor(#{v})", v] }.to_h
     end
diff --git a/lib/onnxruntime/ort_value.rb b/lib/onnxruntime/ort_value.rb
index 3e86175..e5ed719 100644
--- a/lib/onnxruntime/ort_value.rb
+++ b/lib/onnxruntime/ort_value.rb
@@ -29,13 +29,10 @@ def tensor?
     end
 
     def data_type
-      type = FFI::OnnxType[value_type]
-
-      if type == :tensor
-        elem_type = FFI::TensorElementDataType[element_type]
-        "tensor(#{elem_type})"
-      else
-        Utils.unsupported_type("ONNX", type)
+      @data_type ||= begin
+        typeinfo = ::FFI::MemoryPointer.new(:pointer)
+        Utils.check_status FFI.api[:GetTypeInfo].call(out_ptr, typeinfo)
+        Utils.node_info(typeinfo)[:type]
       end
     end
 
diff --git a/lib/onnxruntime/utils.rb b/lib/onnxruntime/utils.rb
index fa6989b..4d068cd 100644
--- a/lib/onnxruntime/utils.rb
+++ b/lib/onnxruntime/utils.rb
@@ -52,5 +52,57 @@ def self.tensor_type_and_shape(tensor_info)
 
       [type.read_int, dims]
     end
+
+    def self.node_info(typeinfo)
+      onnx_type = ::FFI::MemoryPointer.new(:int)
+      check_status api[:GetOnnxTypeFromTypeInfo].call(typeinfo.read_pointer, onnx_type)
+
+      type = FFI::OnnxType[onnx_type.read_int]
+      case type
+      when :tensor
+        tensor_info = ::FFI::MemoryPointer.new(:pointer)
+        # don't free tensor_info
+        check_status api[:CastTypeInfoToTensorInfo].call(typeinfo.read_pointer, tensor_info)
+
+        type, shape = Utils.tensor_type_and_shape(tensor_info)
+        {
+          type: "tensor(#{FFI::TensorElementDataType[type]})",
+          shape: shape
+        }
+      when :sequence
+        sequence_type_info = ::FFI::MemoryPointer.new(:pointer)
+        check_status api[:CastTypeInfoToSequenceTypeInfo].call(typeinfo.read_pointer, sequence_type_info)
+        nested_type_info = ::FFI::MemoryPointer.new(:pointer)
+        check_status api[:GetSequenceElementType].call(sequence_type_info.read_pointer, nested_type_info)
+        v = node_info(nested_type_info)[:type]
+
+        {
+          type: "seq(#{v})",
+          shape: []
+        }
+      when :map
+        map_type_info = ::FFI::MemoryPointer.new(:pointer)
+        check_status api[:CastTypeInfoToMapTypeInfo].call(typeinfo.read_pointer, map_type_info)
+
+        # key
+        key_type = ::FFI::MemoryPointer.new(:int)
+        check_status api[:GetMapKeyType].call(map_type_info.read_pointer, key_type)
+        k = FFI::TensorElementDataType[key_type.read_int]
+
+        # value
+        value_type_info = ::FFI::MemoryPointer.new(:pointer)
+        check_status api[:GetMapValueType].call(map_type_info.read_pointer, value_type_info)
+        v = node_info(value_type_info)[:type]
+
+        {
+          type: "map(#{k},#{v})",
+          shape: []
+        }
+      else
+        Utils.unsupported_type("ONNX", type)
+      end
+    ensure
+      release :TypeInfo, typeinfo
+    end
   end
 end
diff --git a/test/model_test.rb b/test/model_test.rb
index 6f98b2e..c79d8f0 100644
--- a/test/model_test.rb
+++ b/test/model_test.rb
@@ -226,7 +226,10 @@ def test_run_with_ort_values
     sess = OnnxRuntime::InferenceSession.new("test/support/lightgbm.onnx")
     x = OnnxRuntime::OrtValue.ortvalue_from_numo(Numo::SFloat.cast([[5.8, 2.8]]))
     output = sess.run_with_ort_values(nil, {input: x})
+    assert_equal true, output[0].tensor?
     assert_equal "tensor(int64)", output[0].data_type
+    assert_equal false, output[1].tensor?
+    assert_equal "seq(map(int64,tensor(float)))", output[1].data_type
   end
 
   def test_invalid_rank