This thesis proves the details about the U.S.–Japan defense cooperation before the Guidelines for Defense Cooperation between the U.S. and Japan (1978) were created. Japan and the U.S. periodically made CJOEPs (Combined Joint Outline Emergency Plan, before 1964/Coordinated Joint Outline Emergency Plan, after 1964) and contingency plans (including “Hakone” between the U.S. Navy in Japan and the Maritime Self Defense Force). The contents of the CJOEPs and the details about how to make the plans are successfully shown in this thesis. Accordingly, the draft of the CJOEP in 1955 mentioned that “unified command, under a Combined Force Commander, will be established over all U.S. and Japanese forces in Japan. The Combined Force Commander… will exercise command through a combined and joint command structure.” Considering the U.S.–Japan relationship at that time, the Combined Force Commander was assumed to be an American. However, it was finally changed without mentioning the Combined Force Commander. This means that Japan finally succeeded in avoiding the apparent mention of a “secret agreement” in the CJOEP. From this, we can see Japan’s strong will to retain its own sovereignty and independence. The Japan Self Defense Forces (JSDF) and the U.S. Forces in Japan (USFJ) not only made those plans but also conducted map exercises so that they could examine the plans and reflect the results of the exercises in the next year’s plans. This thesis shows the details of the exercises called “FUJI” in 1957 and “MAPLE LEAF” in 1958, and how the JSDF and USFJ conducted them. The joint exercises between the JSDF and the USFJ that took place before the Guidelines are also shown. The fact that the Japan Ground SDF and Air SDF engaged in them with their counterparts should be especially emphasized, because existing studies have not proven this with first-hand materials yet. In addition, this thesis could prove the fact that there were some institutions for defense cooperation between the JSDF and the USFJ at various levels including the Combined Planning Committee, which was established for making the CJOEPs and Combined Planning Groups that were in each service, and set up for making contingency plans based on the CJOEPs. This result means that the defense cooperation during this period was greater than previously thought. Moreover, this leads to two implications: first, the aspect of “symmetric alliance” in the U.S.–Japan Security Arrangement existed earlier and was more substantial; second, the institutionalization of the arrangement was more developed than previous studies have indicated.