In system development, it is an important issue how to reuse modules of existing systems. However, in a situation where systems similar to each other but customized differently are required continually, reuse of the modules becomes more difficult because relations among their functions are complicated. To this problem, we aim to establish a technology for extracting a model of functional structure in design documents of the existing system and utilizing it for development of other new systems. In this paper, we introduce our trial approach to extract the model and also discuss how to use a domain ontology in that process.