Taira Fureai Onsen Center Yuraku

They're closing today at 4 PM until January 5th, so it was packed (by local standards), hence the photos are from the internet, but from different seasons. Here, as is common in the local mountains, there are no fences (in the women's section either). The water is alkaline, the rotenburo is gensen-kakenagashi, indoors it circulates. The rotenburo itself is small, but there usually aren't many people in it. Tattoos are allowed, all amenities provided, costs 720 yen, there's a small restaurant.

Very similar to Kuroba Onsen (just above), except the view from this one is even more beautiful since it faces a dam. Also has everything except fences — tranquility, relaxation, mountains, river. They serve local noodles (specifically soba, the specialty of the local mountain villages).