Firms of oligopoly market usually behave similarly, because of the particular structure of the market in which firms interact with each other intensely. When one firm behaves, the others react immediately, and therefore, as one firm raises prices, his rivals, in their rationality, bound to behave in the way which is parallel to what the firm did. And this situation can be justified by "Game Theory". Even the conscious parallel behavior ends up in noncompetitive circumstances, but without any agreement by the firms, there's no reason to punish them. So it's difficult for Fair Trade Commission to prove that there exists agreement between firms in the oligopoly market. The theory of Facilitating Devices is exercised in practice due to the difficulty of proving. When facilitating devices exist, Fair Trade Commission regards them as evidence to prove that there is cartel agreement between firms, and concludes that the firms violate the Competition Law. However, there are some doubts whether facilitating devices could be immediately regarded as the existence of agreement or mutual understanding. CPC Corporation and FPCC have similar price in recent years. Does this situation results from the preannouncement of price changes or just because the unique structure of oligopoly market? The Fair Trade Commission considered it as concerted action because of the mutual understanding of these two companies. But what does mutual understanding mean? Could preannouncement of price changes be regarded as mutual understanding? And why are the similar behaviors of these companies not conscious parallel behaviors? Those are problems worth to be discussed.