Kotan Onsen Rotenburo is a free, and open air hot spring.The water is clear and warm without being too hot to be able to get into right away. If it indeed gets too hot, you can always take a dip in the lake right next to the bath. The bath is a rock bath with a big rock separating the mens and womans bath. Even though the rock is big, it is not big enough to hide the womans bath and everyone in it from the mens bath and everyone in the mens baths. Although Kotan Onsen Rotenburo is not officially a mixed hot spring, it is just like being in a mixed hot spring.
As far as I remember, there are segregated shelters to undress in.
>From Teshikaga town take route 243 north-west towards Lake Kussharoko. Take the local road 52 going north along the eastern shore of Lake Kussharoko. The first hot spring you will find on your left side between the lake and the road is Kotan Onsen Rotenburo, unless you pass it and end up at Ike-no-yu onsen.