|Open:||6:30 till 22:30|
On Saturday evening I went to Furōsen in downtown Beppu. It's a 100 yen onsen housed in the neighborhood's public hall. There is one big public bath and not much else. Be sure to bring everything you need for bathing yourself. There is not much more I can say about this place, except that it was quite busy, so it is probably better to skip Furōsen if you are looking for a quiet place to relax.
Furōsen means "don't-get-old-spring".
Update: I went to Furōsen on a Wednesday morning and I had the opportunity to take pictures inside.
Facade of Furōsen