diff --git a/README.md b/README.md
index 6ddf1498..11eaf118 100644
--- a/README.md
+++ b/README.md
@@ -686,35 +686,23 @@ subject.Take(100).Count().Subscribe(x => Console.WriteLine(x));
 Parallel.For(0, 1000, new ParallelOptions { MaxDegreeOfParallelism = 10 }, x => subject.OnNext(x));
 ```
 
-This means that the issuance of OnNext must always be done on a single thread. Also, ReactiveProperty, which corresponds to BehaviorSubject in dotnet/reactive, is not thread-safe itself, so updating the value (set Value or call OnNext) must always be done on a single thread.
-
-For converting external inputs into Observables, such as with FromEvent, and when the source of input issues in a multi-threaded manner, it is necessary to synchronize using `Synchronize` to construct the correct operator chain.
-
-Of course, simply surrounding it with `lock` is also effective.
+This means that the issuance of OnNext must always be done on a single thread. For converting external inputs into Observables, such as with `FromEvent`, and when the source of input issues in a multi-threaded manner, it is necessary to synchronize using `Synchronize` to construct the correct operator chain.
 
 ```csharp
-lock(gate)
-{
-    subject.OnNext(value);
-}
+subject.Synchronoize(gate).Take(100).Count().Subscribe();
 ```
 
-Unlike other Subjects, ReactiveProperty's OnNext/Subscribe operations are not thread-safe. Therefore, when using it in a multi-threaded environment, both operations need to be locked.
+In R3, ReplaySubject and BehaviorSubject do not require Synchronize and are thread-safe, including OnNext.
 
-```csharp
-lock(gate)
-{
-    reactiveProperty.Value = value;
-}
+ReactiveProperty is not thread-safe and OnNext, set Value and Susbcribe cannot be called simultaneously. If you need to use it in such a situation, use `SynchronizedReactiveProperty` instead.
 
-lock(gate)
+```csharp
+class MyClass
 {
-    reactiveProperty.Subscribe(observer);
+    public SynchronizedReactiveProperty<int> Prop { get; } = new();
 }
 ```
 
-Regarding Dispose of subscription, it is thread-safe. Additionally, if it's not possible to lock ReactiveProperty itself, you can use `SubscribeOnSynchronize` to apply a lock during Subscribe operations within the operator.
-
 Sampling Timing
 ---
 The `Sample(TimeSpan)` in dotnet/reactive starts a timer in the background when subscribed to, and uses that interval for filtering. Additionally, the timer continues to run in the background indefinitely.
diff --git a/sandbox/ConsoleApp1/Program.cs b/sandbox/ConsoleApp1/Program.cs
index 05d2596f..ec43ff68 100644
--- a/sandbox/ConsoleApp1/Program.cs
+++ b/sandbox/ConsoleApp1/Program.cs
@@ -1,23 +1,10 @@
 using R3;
-using System.Diagnostics;
 
-// https://github.com/Cysharp/R3/issues/232
+var r1 = Observable.Return(1);
+var r2 = Observable.Interval(TimeSpan.FromSeconds(1)).Index();
 
+r1.Concat(r2).Subscribe(Console.WriteLine);
 
-Observable.Concat(
-    Observable.Return("Start"),
-    SomeAsyncTask().ToObservable(),
-    Observable.Return("End")
-).Subscribe(r =>
-{
-    Console.WriteLine("Result:" + r);
-});
+await Task.Delay(TimeSpan.FromDays(1)); // wait
 
 
-Console.ReadLine();
-
-async ValueTask<string> SomeAsyncTask()
-{
-    await Task.Delay(1000);
-    return "result";
-}