diff --git a/src/MessagePack.UnityClient/Assets/Scripts/MessagePack/Resolvers/DynamicObjectResolver.cs b/src/MessagePack.UnityClient/Assets/Scripts/MessagePack/Resolvers/DynamicObjectResolver.cs
index 49e1c821f..c847a3038 100644
--- a/src/MessagePack.UnityClient/Assets/Scripts/MessagePack/Resolvers/DynamicObjectResolver.cs
+++ b/src/MessagePack.UnityClient/Assets/Scripts/MessagePack/Resolvers/DynamicObjectResolver.cs
@@ -1773,7 +1773,7 @@ public static ObjectSerializationInfo CreateOrNull(Type type, bool forceStringKe
                             {
                                 FieldInfo = field,
                                 IsReadable = allowPrivate || field.IsPublic,
-                                IsWritable = allowPrivate || (field.IsPublic && !field.IsInitOnly),
+                                IsWritable = (allowPrivate || field.IsPublic) && !field.IsInitOnly,
                                 StringKey = firstMemberByName ? item.Name : $"{item.DeclaringType.FullName}.{item.Name}",
                             };
                         }
@@ -1956,7 +1956,7 @@ public static ObjectSerializationInfo CreateOrNull(Type type, bool forceStringKe
                     {
                         FieldInfo = item,
                         IsReadable = allowPrivate || item.IsPublic,
-                        IsWritable = allowPrivate || (item.IsPublic && !item.IsInitOnly),
+                        IsWritable = (allowPrivate || item.IsPublic) && !item.IsInitOnly,
                     };
                     if (!member.IsReadable && !member.IsWritable)
                     {
diff --git a/src/MessagePack.UnityClient/Assets/Scripts/Tests/ShareTests/AllowPrivateTest.cs b/src/MessagePack.UnityClient/Assets/Scripts/Tests/ShareTests/AllowPrivateTest.cs
index 505db369e..f27b794e1 100644
--- a/src/MessagePack.UnityClient/Assets/Scripts/Tests/ShareTests/AllowPrivateTest.cs
+++ b/src/MessagePack.UnityClient/Assets/Scripts/Tests/ShareTests/AllowPrivateTest.cs
@@ -157,6 +157,28 @@ internal class InternalClass
             internal InternalEnum EnumProperty { get; set; }
         }
 
+        [MessagePackObject]
+        public class PrivateReadonlyField
+        {
+            public static PrivateReadonlyField WithNullValue { get; } = new PrivateReadonlyField();
+
+            [Key(0)]
+            private readonly string field;
+
+            [SerializationConstructor]
+            public PrivateReadonlyField(string field)
+            {
+                this.field = field ?? "not null";
+            }
+
+            private PrivateReadonlyField()
+            {
+            }
+
+            [IgnoreMember]
+            public string Field => field;
+        }
+
 #if !ENABLE_IL2CPP
 
         [MessagePackObject]
@@ -329,6 +351,15 @@ public void InternalClassWithInternalEnum()
             Assert.Equal(expected.EnumProperty, actual.EnumProperty);
         }
 
+        [Fact]
+        public void PrivateReadonlyFieldSetInConstructor()
+        {
+            PrivateReadonlyField initial = PrivateReadonlyField.WithNullValue;
+            var bin = MessagePackSerializer.Serialize(initial, StandardResolverAllowPrivate.Options);
+            var deserialized = MessagePackSerializer.Deserialize<PrivateReadonlyField>(bin, StandardResolverAllowPrivate.Options);
+            Assert.Equal("not null", deserialized.Field);
+        }
+
 #if !ENABLE_IL2CPP
 
         [Fact]