Upon the passing of Tenkai in 1643, Kokai, residing at Kanei-ji Temple on Mt. Toei, carried on Tenkai's dharma and by placing under his control Mt. Hiei the Grand Head Temple of the Tendai Sect as well as Mt. Nikko where the Nikko Toshogu Shrine dedicated to Ieyasu TOKUGAWA was located, took the helm of the entire Tendai Sect.
1643年（寛永20年）に天海が没するとその法をついで東叡山寛永寺に住し、天台宗の総本山である比叡山・徳川家康を祀る日光東照宮のある日光山を管理下に置いて天台宗一宗を管領するようになった。