Shuzenji Onsen is said to be the oldest hot spring in Izu, and its origins lie in the Tokko-no-yu that springs from the banks of the Katsura River. In the second year of the Daido era, Kobo Daishi visited Shuzenji and was touched by the filial piety of a young boy washing his sick father in the Katsura River. He struck a rock in the river with a Tokko-no-kyo vajra, a Buddhist implement he was carrying, causing a sacred spring to gush forth. When Kobo Daishi bathed in the spring, his father's illness was instantly cured, and it is said that hot spring therapy became widespread.