In recent years, considerable attention has been given to Theory of Games which has been applied to Economics and Biology and so on , as a method of Complex Adaptive System. The purpose of this work is to appear a structure of the similarity of Product Design using that theory. In the first half of this paper, the methodology of Theory of Games has been examined and considered from a historical and technological point of view. Through the case study of general products, the similarity has been analysed and studied by use of the introduced method in the latter half. The similarity led by the purpusing strategy of Product Design has been verified using the expression. It is also found that the discriminating strategy is effective to avoid the similarity.