Why and how did the institutional characteristics often described as the Japanese system of the Japanese enterprise system emerge? Although there is a broad consensus among researchers that Japan's enterprise system emerged between 1930 and 1955, researchers differ with regard to the importance of the prewar system, changes and their magnitude during the wartime planned economy, and the impact of postwar reforms. Okazaki suggested that the origin of the current Japanese economic system can be found in the wartime planned economy. In contrast, this paper suggests that the impact of postwar reform was crucial for establishing the current system. Specifically, that GHQ's “Americanization” of the economic system created a discontinuous transformation of that system. This paper highlights the effects of occupation reform on changes in corporate finance, governance, and industrial organization. In order to clarify the transformation process of the economic system, I use microdata for capital composition, ownership structure, and firm behavior such as depreciation and investment, which I collected from the 126 largest industrial and mining firms.