diff --git a/src/MemoryPack.Generator/DiagnosticDescriptors.cs b/src/MemoryPack.Generator/DiagnosticDescriptors.cs
index c3fd0603..9faf9f8e 100644
--- a/src/MemoryPack.Generator/DiagnosticDescriptors.cs
+++ b/src/MemoryPack.Generator/DiagnosticDescriptors.cs
@@ -316,4 +316,12 @@ internal static class DiagnosticDescriptors
         category: Category,
         defaultSeverity: DiagnosticSeverity.Error,
         isEnabledByDefault: true);
+
+    public static readonly DiagnosticDescriptor GenerateTypeCannotSpeciyToUnionBaseType = new(
+        id: "MEMPACK039",
+        title: "GenerateType cannot be specified for the Union base type itself",
+        messageFormat: "The MemoryPackable object '{0}' cannot specify '{1}'. Because it is Union base type.",
+        category: Category,
+        defaultSeverity: DiagnosticSeverity.Error,
+        isEnabledByDefault: true);
 }
diff --git a/src/MemoryPack.Generator/Extensions.cs b/src/MemoryPack.Generator/Extensions.cs
index bb3cd574..7c64f7cb 100644
--- a/src/MemoryPack.Generator/Extensions.cs
+++ b/src/MemoryPack.Generator/Extensions.cs
@@ -86,13 +86,16 @@ public static bool TryGetMemoryPackableType(this ITypeSymbol symbol, ReferenceSy
         var packableCtorArgs = memPackAttr?.ConstructorArguments;
         generateType = GenerateType.Object;
         serializeLayout = SerializeLayout.Sequential;
+
         if (memPackAttr == null || packableCtorArgs == null)
         {
-            generateType = GenerateType.NoGenerate;
+            var memPackUnionFormatterAttr = symbol.GetAttribute(references.MemoryPackUnionFormatterAttribute);
+            generateType = memPackUnionFormatterAttr != null ? GenerateType.Union : GenerateType.NoGenerate;
             serializeLayout = SerializeLayout.Sequential;
             return false;
         }
-        else if (packableCtorArgs.Value.Length != 0)
+
+        if (packableCtorArgs.Value.Length != 0)
         {
             // MemoryPackable has three attribtues
             // [GenerateType generateType]
diff --git a/src/MemoryPack.Generator/MemoryPackGenerator.Parser.cs b/src/MemoryPack.Generator/MemoryPackGenerator.Parser.cs
index e7379fad..7514d236 100644
--- a/src/MemoryPack.Generator/MemoryPackGenerator.Parser.cs
+++ b/src/MemoryPack.Generator/MemoryPackGenerator.Parser.cs
@@ -534,6 +534,12 @@ public bool Validate(TypeDeclarationSyntax syntax, IGeneratorContext context, bo
                 noError = false;
             }
 
+            if (GenerateType != GenerateType.Union)
+            {
+                context.ReportDiagnostic(Diagnostic.Create(DiagnosticDescriptors.GenerateTypeCannotSpeciyToUnionBaseType, syntax.Identifier.GetLocation(), Symbol.Name, GenerateType));
+                noError = false;
+            }
+
             if (UnionTags.Select(x => x.Tag).HasDuplicate())
             {
                 context.ReportDiagnostic(Diagnostic.Create(DiagnosticDescriptors.UnionTagDuplicate, syntax.Identifier.GetLocation(), Symbol.Name));
diff --git a/src/MemoryPack.Unity/Assets/Plugins/MemoryPack/Runtime/MemoryPack.Generator/MemoryPack.Generator.Roslyn3.dll b/src/MemoryPack.Unity/Assets/Plugins/MemoryPack/Runtime/MemoryPack.Generator/MemoryPack.Generator.Roslyn3.dll
index 50a20fe2..8beee2b5 100644
Binary files a/src/MemoryPack.Unity/Assets/Plugins/MemoryPack/Runtime/MemoryPack.Generator/MemoryPack.Generator.Roslyn3.dll and b/src/MemoryPack.Unity/Assets/Plugins/MemoryPack/Runtime/MemoryPack.Generator/MemoryPack.Generator.Roslyn3.dll differ
diff --git a/src/MemoryPack.Unity/Assets/Scripts/MemoryPackObjects/MemberKinds.cs b/src/MemoryPack.Unity/Assets/Scripts/MemoryPackObjects/MemberKinds.cs
index 65e1d79e..c58f6fa8 100644
--- a/src/MemoryPack.Unity/Assets/Scripts/MemoryPackObjects/MemberKinds.cs
+++ b/src/MemoryPack.Unity/Assets/Scripts/MemoryPackObjects/MemberKinds.cs
@@ -38,6 +38,12 @@ public partial class MemberKindsAllUnmanaged
 
         int[] kArray = new int[1];
 
+        public MemberKindsAllUnmanaged(int c, int d)
+        {
+            this.C = c;
+            this.D = d;
+        }
+
         public ref int K
         {
             get { return ref kArray[0]; }
diff --git a/tests/MemoryPack.Tests/Models/Union.cs b/tests/MemoryPack.Tests/Models/Union.cs
index 83dea6a3..2bf8b3f7 100644
--- a/tests/MemoryPack.Tests/Models/Union.cs
+++ b/tests/MemoryPack.Tests/Models/Union.cs
@@ -144,6 +144,7 @@ public partial class ForExternalUnionFormatter3
 public partial class NoraType
 {
     public IForExternalUnion? ExtUnion { get; set; }
+    public UnionAbstractClass? AbstractUnion { get; set; }
 }
 
 // Union for record