diff --git a/src/VoicevoxCoreSharp.Core/Synthesizer.cs b/src/VoicevoxCoreSharp.Core/Synthesizer.cs
index efe41fb..208e7ae 100644
--- a/src/VoicevoxCoreSharp.Core/Synthesizer.cs
+++ b/src/VoicevoxCoreSharp.Core/Synthesizer.cs
@@ -68,16 +68,22 @@ public ResultCode LoadVoiceModel(VoiceModelFile voiceModel)
             }
         }
 
-        public ResultCode UnloadVoiceModel(string modelId)
+        public ResultCode UnloadVoiceModel(VoiceModelId modelId)
         {
             unsafe
             {
-                fixed (byte* ptr = NativeUuid.ToUUIDv4ByteArray(modelId))
+                fixed (byte* ptr = NativeUuid.ToUUIDv4ByteArray(modelId.Id))
                 {
                     return CoreUnsafe.voicevox_synthesizer_unload_voice_model((VoicevoxSynthesizer*)Handle, ptr).FromNative();
                 }
             }
         }
+        
+        // For backward compatibility
+        public ResultCode UnloadVoiceModel(string modelId)
+        {
+            return UnloadVoiceModel(new VoiceModelId(modelId));
+        }
 
         public bool IsGpuMode
         {
@@ -90,16 +96,22 @@ public bool IsGpuMode
             }
         }
 
-        public bool IsLoadedVoiceModel(string modelId)
+        public bool IsLoadedVoiceModel(VoiceModelId modelId)
         {
             unsafe
             {
-                fixed (byte* ptr = NativeUuid.ToUUIDv4ByteArray(modelId))
+                fixed (byte* ptr = NativeUuid.ToUUIDv4ByteArray(modelId.Id))
                 {
                     return CoreUnsafe.voicevox_synthesizer_is_loaded_voice_model((VoicevoxSynthesizer*)Handle, ptr);
                 }
             }
         }
+        
+        // For backward compatibility
+        public bool IsLoadedVoiceModel(string modelId)
+        {
+            return IsLoadedVoiceModel(new VoiceModelId(modelId));
+        }
 
         public string MetasJson
         {
diff --git a/src/VoicevoxCoreSharp.Core/VoiceModel.cs b/src/VoicevoxCoreSharp.Core/VoiceModel.cs
index f47b34a..b824a12 100644
--- a/src/VoicevoxCoreSharp.Core/VoiceModel.cs
+++ b/src/VoicevoxCoreSharp.Core/VoiceModel.cs
@@ -34,21 +34,21 @@ public class VoiceModelFile : IDisposable
         internal VoiceModelFileHandle Handle { get; private set; }
         private bool _disposed = false;
 
-        public string Id { get; private set; }
+        public VoiceModelId Id { get; private set; }
 
         public string MetasJson { get; private set; }
 
         private VoiceModelFile()
         {
             Handle = new VoiceModelFileHandle(IntPtr.Zero);
-            Id = string.Empty;
+            Id = new VoiceModelId(string.Empty);
             MetasJson = string.Empty;
         }
 
         private unsafe VoiceModelFile(VoicevoxVoiceModelFile* modelHandle, string id, string metasJson)
         {
             Handle = new VoiceModelFileHandle(new IntPtr(modelHandle));
-            Id = id;
+            Id = new VoiceModelId(id);
             MetasJson = metasJson;
         }
 
diff --git a/src/VoicevoxCoreSharp.Core/VoiceModelId.cs b/src/VoicevoxCoreSharp.Core/VoiceModelId.cs
new file mode 100644
index 0000000..44b5e53
--- /dev/null
+++ b/src/VoicevoxCoreSharp.Core/VoiceModelId.cs
@@ -0,0 +1,16 @@
+using System;
+
+namespace VoicevoxCoreSharp.Core
+{
+    public record VoiceModelId(string Id)
+    {
+        // Target framework is netstandard2.0, so init-only properties not available
+        public string Id { get; } = Id;
+
+        public static implicit operator string(VoiceModelId id) => id.Id;
+        
+        public static implicit operator VoiceModelId(string id) => new VoiceModelId(id);
+        
+        public override string ToString() => Id;
+    }
+}
\ No newline at end of file
diff --git a/tests/VoicevoxCoreSharp.Core.Tests/SynthesizerTest.cs b/tests/VoicevoxCoreSharp.Core.Tests/SynthesizerTest.cs
index 2aa1938..8c6b803 100644
--- a/tests/VoicevoxCoreSharp.Core.Tests/SynthesizerTest.cs
+++ b/tests/VoicevoxCoreSharp.Core.Tests/SynthesizerTest.cs
@@ -49,6 +49,7 @@ public void VoiceModelLoaded()
             VoiceModelFile.Open(Consts.SampleVoiceModel, out var voiceModel);
             synthesizer.LoadVoiceModel(voiceModel);
 
+            // The IsLoadedVoiceModel and UnloadVoiceModel methods should work with the VoiceModelId type
             Assert.True(synthesizer.IsLoadedVoiceModel(voiceModel.Id));
             synthesizer.UnloadVoiceModel(voiceModel.Id);
             Assert.False(synthesizer.IsLoadedVoiceModel(voiceModel.Id));
diff --git a/tests/VoicevoxCoreSharp.Core.Tests/VoiceModelIdTest.cs b/tests/VoicevoxCoreSharp.Core.Tests/VoiceModelIdTest.cs
new file mode 100644
index 0000000..1bfb146
--- /dev/null
+++ b/tests/VoicevoxCoreSharp.Core.Tests/VoiceModelIdTest.cs
@@ -0,0 +1,38 @@
+using System;
+using Xunit;
+
+namespace VoicevoxCoreSharp.Core.Tests
+{
+    public class VoiceModelIdTest
+    {
+        [Fact]
+        public void CreateAndConvertToString()
+        {
+            var id = new VoiceModelId("test-id");
+            Assert.Equal("test-id", id.Id);
+            Assert.Equal("test-id", id.ToString());
+            
+            // Test implicit conversion to string
+            string idString = id;
+            Assert.Equal("test-id", idString);
+        }
+        
+        [Fact]
+        public void CreateFromString()
+        {
+            VoiceModelId id = "test-id";
+            Assert.Equal("test-id", id.Id);
+        }
+        
+        [Fact]
+        public void Equality()
+        {
+            var id1 = new VoiceModelId("test-id");
+            var id2 = new VoiceModelId("test-id");
+            var id3 = new VoiceModelId("different-id");
+            
+            Assert.Equal(id1, id2);
+            Assert.NotEqual(id1, id3);
+        }
+    }
+}
\ No newline at end of file
diff --git a/tests/VoicevoxCoreSharp.Core.Tests/VoiceModelTest.cs b/tests/VoicevoxCoreSharp.Core.Tests/VoiceModelTest.cs
index 954743b..101ac81 100644
--- a/tests/VoicevoxCoreSharp.Core.Tests/VoiceModelTest.cs
+++ b/tests/VoicevoxCoreSharp.Core.Tests/VoiceModelTest.cs
@@ -13,7 +13,7 @@ public void Open()
             Assert.Equal(ResultCode.RESULT_OK, openResult);
             Assert.NotNull(voiceModel);
 
-            Assert.NotEmpty(voiceModel.Id);
+            Assert.NotEmpty(voiceModel.Id.Id);
             Assert.NotEmpty(voiceModel.MetasJson);
         }
 
@@ -23,10 +23,10 @@ public void AccessMemberAfterDisposed()
             var _ = VoiceModelFile.Open(Consts.SampleVoiceModel, out var voiceModel);
             using (voiceModel) { }
 
-            Assert.NotEmpty(voiceModel.Id);
+            Assert.NotEmpty(voiceModel.Id.Id);
             Assert.NotEmpty(voiceModel.MetasJson);
 
-            Assert.True(voiceModel.Id.Length > 0);
+            Assert.True(voiceModel.Id.Id.Length > 0);
             Assert.True(voiceModel.MetasJson.Length > 0);
         }
     }