Recently Japanese sogo shosha has begun attracting the interest of scholars. Sogo Shosha is a very unique economic organization which has developed fully only in Japan. It handles wide variety of product lines, operate all over the world, and performs various kinds of functions. So far a lot of studies of sogo shosha has been done, and they have stressed the importance of its overseas operations. In spite of the importance of its world-wide operations based upon branch activities, there is no systematic study of activities of foreign branches. There appear several questions, related with activities of overseas branches of sogo shosha. Why and how does headquarters open a particular branch? What is the relationship between the opening of overseas branches and the other strategies of sogo shosha including diversification of product lines and integration of functions. How are they controlled? What kinds of managerial problems does they have, and how are these problems solved? These questions have remained unanswered. This study aims to answer these questions on sogo shosha's overseas operations, as well as its development as a whole. For this objective this study traced the development and activities of the San Francisco and Seattle Branches of Mitsubishi Shoji Kaisha in the period between 1918 when the Seattle branch opened and the outbreak of World War II, which ended the operations of both branches, using the company's original records, which were confiscated by the United States government just after Pearl Harbor.