In software development, it has been pointed out that software engineers must pay attention to software requirements definition. One of the important problems in software engineering is to rationalize the processes from requirements definition to design. Computer tools are most useful and efficient for this purpose. This paper proposes a computer-aided software design system (CASDS), which supports software engineers with a series of structural modeling. As is well-known in systems planning, structural modeling helps to extract concepts from many fuzzy requirements. This system contains three structural modeling methods. They are used 1) to determine functional terms from fuzzy software requirements, 2) to obtain modules by structuring the functions with respect to the data flows, and 3) to make a program skeleton by imposing control flows on the functional elements obtained by breaking down the modules.