Iou-zan Kokubun-ji is a Shingon temple called Saishō-in. This temple was originally built during the Tempyō era (729 to 749) as a provincial temple in Musashi-no-kuni (Musashi Province) at the request of the Emperor Shōmu, who is known for constructing the Great Buddha of Tōdai-ji Temple in Nara. In 1333 (the third year of Genkō) it was rebuilt after being burnt down by Nitta Yoshisada's assault on Kamakura.