A series of political arguments during negotiation and ratification process of the London Naval Treaty of 1930 was one of the most serious domestic political situations in modern Japan within the Cabinet, the Foreign Ministry, the Navy, the political parties such as Minseito and Seiyukai, the Privy Council, the Kizoku-in, the Army and civilian right wingers. The London treaty fight deeply affected the fate of Japan. This study examines the policy argument over the treaty, focusing on Prime Minister Hamaguchi who was a main player to press for the treaty. It has been pointed out that Hamaguchi was moved by budgetary concerns but those who were against ratification of the treaty like Admiral Kato Kanji, were opposed him from military point of view. My interpretation, however, is that there are other significant factors: Hamaguchi's and Kato's argument over how Japanese policy toward the United States and China should be, or the future of Japan should be as a member of international community. Hamaguchi and his opponents' ideas were so different, and that was one of the reasons for their serious conflict. After all the political argument within Japan, it can be said Japan's new state system operated by political parties including the Navy, the Army, and Privy Council was eventually working under the Hamaguchi Cabinet. At the same time, ratification of the London Naval Treaty of 1930 made it possible for Japan to become one of the leading countries in international society, along with the United States and Britain.
