This chapter examines the factors that drove the Japanese government to announce the arms trade ban policy in 1967 and to uphold it for many years. It could be argued that the government announced the policy under the influence of a pacifist norm. However, the chapter argues that successive Liberal Democratic Party (LDP) governments tried to avoid provoking debates on security-related policy due to strong criticism by the opposition parties. By interpreting Article 9 of the Constitution rigidly, the parties argued that exporting arms would be equivalent with Japan using force. Although the government rejected this argument, the ideational distribution on the domestic political scene drove the LDP government to announce the arms trade ban policy so as to appease the left. Unexpectedly, the policy became institutionalised, thus constraining Japan’s security option until it was eased in 2011.