It is important to assure the security of systems in the course of development. However, lack of requirements analysis method to integrate security functional requirements analysis and validation in upper process often gives a crucial influence to the system dependability. For security requirements, even if extraction of menaces was completely carried out, insufficient countermeasures do not satisfy the security requirements of customers.In this paper, we propose a method to describe security cases based on the security structures and threat analysis. The security structure of the method is decomposed by the Common Criteria (ISO/IEC15408).