Oto-gawa River in Wakayama is famous for its onsen springing out from the dug bottom of the river. They dam up the river and 'Sennin Buro' comes out from December to February. Wearing bathing suits, men and women bathe together and enjoy onsen in the great nature all together. And more, it's free!
Kawayu Onsen Sennin Buro
Kawayu Hongu-cho, tanabe-shi, wakayama