This report aims to build a better understanding of status quo and prospect of methane mitigation in oil and gas sector of Japan. The related policies and practices, which have been taken or considered by the national government, industrial association and major companies, are reviewed and summarized accordingly. A short summary is also provided for methane legislation and policy of oil and gas sector in the U.S. and EU. Based on this, the possible impact of methane policy of the U.S. and EU on Japan’s energy security and decarbonization is identified. Another issue to concern in this report is about the role and potential for Japan to assist the developing Asian countries in methane mitigation during the transitional period and toward full decarbonization of energy sector in the long run. For this purpose, this study also reviews methane mitigation policies and practices in energy sector of several ASEAN member states, including Malaysia, Brunei, Thailand, Indonesia and Vietnam, to identify the existing policy gaps and their cooperation opportunities with Japan.