The article explains how and why Japan and ROK (South Korea) cooperate or alienate in Northeast Asia analyzing their foreign policies in the three triangles; Japan-DPRK (North Korea)-ROK, Japan-U.S.-ROK, and Japan-PRC (China)-ROK trilateral relations.
　　Japan and ROK have divergent perceptions and make different foreign policies particularly on the issues of DPRK nuclear weapons and long-range missile developments, alliance politics with the U.S., security relations including PRC, and so forth.
　　Meanwhile, both of them are similar in terms of non-nuclear, non-hegemonic power, and democratic system in the region. There can be many common interests including potential ones between Japan and ROK in envisioning a future multilateral common security institution in Northeast Asia.