Confidentiality is one of the most important requirements of information protection systems. The access control technique has been used to provide confidentiality, but it has fundamental problems in that it cannot prevent violations of confidentiality committed by authorized users. Information flow control is a technique introduced to resolve such problems, and many approaches based on programming languages have been proposed. However, it is not easy for a programmer to implement the technique at the source code level. Furthermore, the practicality of information flow control is difficult to demonstrate because it does not provide control over programs that have already been developed. This paper proposes a method that enables a practical information flow control through using a protected area, a separate part of computer system storage. Case studies are given to show its usefulness.