This paper explores the change in the security portfolio of the holding company of Mitsubishi Zaibatsu (Mitsubishi Partnership) and its implication on the profitability of that company. First we confirmed that Mitsubishi Partnership substantially altered the portfolio from 1925 to 1935, especially in the early 1930s. Then we decomposed the profitability change into that due to the changes in the returns on the individual securities which had existed in the portfolio and that due to the change in the portfolio. We found that whereas the former factor was larger, the portfolio factor was also substantial and positively contributed to the profitability. It is suggested that the zaibatsu holding company secured profitability thorough management of the portfolio as well as thorough governance of the companies in which it invested.