A priest of the Tendai school in Japan. He studied under Jikaku, the third chief priest of Enryaku-ji, the head temple of the Tendai school on Mount Hiei. He is said to have excelled in the performance of the esoteric prayer rituals. In 865 he founded Mudo-ji temple on Mount Hiei, in which a statue of the wisdom king Immovable was enshrined. In 866 he petitioned Emperor Seiwa to grant the title "great teacher" to Dengyoand Jikaku. This was the first instance of the title "great teacher" being bestowed in Japan. So'ois also regarded as the founder of the Tendai practice of visiting the assigned shrines and halls on Mount Hiei every day for one thousand days (known as kaiho-gyo, "making a round on the mountain").