The purpose of this essay is to make clear the roles of company groups, trade associations, and the government for the competitive power or growth of Japanese companies. It takes up the case of the petrochemical industry, which presents the most suitable data on the point at issue. The first half of this paper examines the historical outline of Japan's petrochemical industry, which has gone through the following three periods : 1. The period of the beginning of home production (1957-1964); 2. The period of the large-sized investment in plant and equipment (1965-1973); 3. The period of the reorganization of the industry after the first and second Oil Shock (1974-). The latter half investigates the functions of company groups (Mitsui, Mitsubishi, and Sumitomo), trade association (Sekiyu Kagaku Kogyo Kyokai, the Petrochemical Industry Association), and the government (the Ministry of International Trade and Industry). After due criticism against the commonly accepted theories, I draw two conclusions as follows : 1. Company groups, trade associations, and the government made many-sided contributions to the competitive power or growth of Japanese companies; 2. Nevertheless, the functions of company groups, trade associations, and the government was within the limit of supplementing organizational capability of Japanese companies.