Kami-no-yu

Kaminoyu (上の湯) is a small, unstaffed communal bath on a roadside in the Yamaguchi part of Shima Onsen, easy to miss unless you're looking for it. The men's and women's sides each have two indoor tubs with the spring water kept flowing through them (kakenagashi), one hotter and one kept lukewarm, so you can move between them. There's no shower or washing area; you rinse with a bucket before getting in. Entry is free, with a small donation box for anyone who wants to leave something. Local residents take turns keeping the place clean and running it on a rotation, and reviewers keep noting how well kept and quietly loved this small bath is.