This paper overviews the industrial organization in Japan between the two World Wars. In this period, the change in the industrial structure gave a substantial impact on the industrial organization. On one hand, development of the heavy industries, which had been highly concentrated, raised the average level of market concentration (between effect). On the other hand, market concentration of each industry declined, in particular for the heavy industries (within and covariance effects). Decline of market concentration in each industry reflected the change in firm dynamics. While "natural selection" shaking out inefficient firms weakened, new entries continued in many industries. This new pattern of firm dynamics was associated with the activities of cartels, which proliferated in this period. Cartels indeed restricted competition and enhanced profitability of industries in the short run, which in turn undermined natural selection and induced new entries in the long run.