Japan’s diplomacy during the Cold War has often been described as passive on international political issues. In line with the so-called Yoshida Doctrine, Japan prioritized its economic development and largely entrusted its defense to the United States. While keeping a low profile on international security and political affairs, Japan contributed to peace and stability by promoting regional economic development through aid, trade and investment. Yet, an examination of Tokyo’s Southeast Asia diplomacy shows that, starting from the 1960s, Japan became gradually involved in efforts to settle regional conflicts. One example is Japan’s diplomacy related to the Jakarta conference of May 1970, which brought together Asian-Pacific states for the purpose of resuming peace in Cambodia after the Vietnam War’s front had expanded to that country. Despite this being one of the initial cases of Japan enlarging the scope of its diplomacy by actively engaging in peace efforts, little has been written on it. Drawing on declassified diplomatic documents, this paper seeks to clarify Japan’s role and objectives in embarking on this diplomatic endeavor, its effectiveness and the impact on Tokyo’s regional diplomacy. This unprecedented case of a Japanese post-World War II participation in a regional conference to solve a conflict offers important indications on the features and limitations of Japan’s initial steps in this area of its diplomacy.