This article examines the relationship between the Mutual Defense Treaty (the Treaty) between the United States of America and the Republic of China on the one hand and Japan on the other from four different aspects. Most of the Japanese scholars have so far focused their researches almost exclusively on the Treaty in terms of its background, settlements and applications, while few studies cared to look at its relationship with Japan. Before and after the Treaty was signed, Asahi Shimbun and Yomiuri Shimbun had covered this event from time to time. However, as far as this author has explored, the Japanese government did not issue any official comments on the Treaty within this period of time. After the signing of the Treaty, the Japanese embassies in the US, Taiwan and etc., did send prime time reports about other countries' media coverage of it to the Ministry of Foreign Affairs of Japan, which played a significant role in shaping the Japanese government's understanding of the Treaty. On the basis of a general review of the Japanese Diet's meeting minutes, it has revealed that the Treaty had been discussed mainly from four aspects: first and foremost, whether or not to recognize and establish foreign relations with mainland China or Taiwan; second, issues about the Far Eastern Clause and Taiwan clause; third, the ownership of Taiwan; and fourth, the reversion of Okinawa. The Treaty has since had certain degree of influences on Japan's domestic and foreign affairs. Nevertheless, when it comes to Japan's foreign policy on Taiwan and mainland China, the Japanese government has always looked to the US policies as a bellwether. The Mutual Defense Treaty between the United States of America and the Republic of China and Treaty of Mutual Cooperation and Security between Japan and the United States of America were both results of the Cold War.