Nyuto Onsen is a remote, hot spring enclave in the mountains along a river. There’s a variety of hot spring baths, both indoors and outdoors. Very rustic and charming in scenic, natural surroundings. Some people live there for weeks or months to cure themselves. The right photo shows streams of hot water to massage your shoulders or back.
I remember Nyuto Onsen because that’s where I had my first mixed-bathing experience. Even today, bathers are not allowed to wear any bath towel or swimsuit in the bathing area and baths. I was still young and naive and disappointed because the women in the baths were my grandmother’s age. 
Totally different experience from Dogo Onsen in Matsuyama, Ehime Prefecture where the men’s bathhouse was more like a social hall with naked, old men just sitting and chatting around the communal bath with their feet in the bath. 
Nyuto Onsen is quite out of the way. From Tazawako Station on the Akita Shinkansen, it’s a 50-min. bus ride.
Official: http://www.nyuto-onsenkyo.com/