Anoi Onsen is a rustic ryokan deep in valley of the mountain area. A river passes thro' the ryokan area and the main characteristic is it uses oil lamps instead of electricity.
Here is the website [in Japanese only]..
We stayed on the G/F of this wooden house - I saw the pic of it in a magazine and I liked it so much, that's how we decided to search for this place !!! The ryokan has one main building and a few detached houses. This is one of them.
[pic from Aoni Onsen website]
They have 4 onsens - 2 of them are mixed bath - that means both men and women share the same onsen! The one in the pic is called "Rotennyuyoku", dates back to 70 years ago. It's a pity it's not a real open air onsen but a covered one.
This is called "Ryujin". Also a mixed bath. [pic from Aoni Onsen website]
It is called "Uchiyu".[pic from Aoni Onsen website]
It is called "Kenroku". [pic from aoni Onsen website]