How to write a CSV file with Japanese characters?

Here is my code:

  System.IO.FileStream f = new System.IO.FileStream("C:\\temp.csv", System.IO.FileMode.Create);
  byte[] bytes = System.Text.Encoding.UTF8.GetBytes("あ");
  f.Write(bytes, 0, bytes.Length);

The character saved in CSV file is in Japanese style but is not the input character.

Could experts here show me the way to solve this problem?

Many thanks.
eHanoi