He participated in the plan led by his father, Tadamune, to assassinate MINAMOTO no Yoshitomo; moreover, he also killed Masakiyo KAMATA, Yoshitomo's closest Vassal and Kagemune's Gikyodai (relationship associated with each other as brother), during the assassination.
父、忠致の源義朝暗殺計画に加担し、実行の際には義朝の第一の郎従であり、自分の義兄弟でもある鎌田政清を斬っている。