In this paper, the authors design a file security function on Windows O.S (Operating System). Whenever users use Windows O.S, they need to protect some file data. This paper designs these security protection functions. This paper proposes two security functions on Windows O.S. One is file security. The other is directory access protection. To check the proposed functions well, the authors experiment the above functions on the Windows O.S. By this experiment, they confirmed that the proposed function worked well.