The purpose of this paper is to provide a review of modularization by Japanese firms on the basis of the analytical framework of product architecture. Japanese firms try to create system control modules, and attempt the rationalization of the design. However, they always design systems using the basic concept of integral type architecture. Modularization by Japanese firms is not a random design. It is the result of a design strategy that simultaneously achieves the optimization and rationalization of the product design. This paper reviews the product modularization processes of Japanese firms based on the analytical framework of product architecture theory. Basically, Japanese manufacturing firms have created system control modules to achieve rational product designs. However, conceptually, the system was always designed as an integral type. We conclude that, to Japanese firms, modularization is not a random choice of design but is the result of design strategies to simultaneously meet the optimization and rationalization of the product design.