A security system design supporting tool and method are disclosed, in which security requirements (PP) and security specifications (ST) used for designing a product or a system (TOE) based on CC requirements can be prepared efficiently and uniformly even by ordinary designers other than specialists. In a security system design supporting method, registered PPs and past PP/ST generation cases are so structured as to reuse and/or reference as templates, a draft is automatically generated, and the draft thus generated is additionally modified or corrected by partial automatic generation utilizing a database of past generation cases and partial case accumulated in the generation process thereof.