This is a fantastic onsen. If you are in the Nasushiobara area for just a short time and you want to go to an onsen this is our pick so far.
Its not so hard to find. The hotel itself is on the way to Shiobara on highway 400. Its not very attractive from the road. There is not much room there due to the very steep valley and the hotel is jammed in behind the road. There is a small parking area. If the parking area is full there is an old disused section of highway 50 meters down the hill where you can park and walk (carefully) up to the hotel.
Go into the hotel and tell them you want an onsen. The price is 800 yen per person. Then you have to walk to the onsen. Across the road from the onsen just up the hill from the carpark there is a small path, leaving the road, going down the hill. The onsen itself is all the way at the bottom of the hill by the river. Its peaceful, quiet, by the river and has great views. There is a private women's onsen, a large public pool that I'm assuming is mixed though I have only ever seen men in it (except for when its very quiet and my wife sneaks in). Then there are two quiet little pools right by the river. The little pools are not so great in my opinion because they are always too hot and too cold though one of them is very "by the river".
This is easily the number one pick for natural beauty onsen.