Seikōkan

Seikokan keeps things simple: just two indoor baths, one built of rock and tile, one entirely of unpainted hinoki wood. Both draw straight from a self-flowing radium spring, no recirculation, no chlorine, just a bit of reheating, so the water stays soft and gentle on the skin. Because the spring's own flow is limited, the baths are compact rather than grand. The building is the real story: a three-story wooden ryokan the inn says has been open for over 300 years, registered as a national tangible cultural property. It's a member of the association for secluded hot-spring inns, Hitou o Mamoru Kai (日本秘湯を守る会). Temple carpenters finished each room with its own choice of wood and joinery, the stairs are steep, the layout can feel like a small maze, and there's no elevator between the floors and the ground-level baths. It's stay-only, with no day-use bathing.