When you have to drive this far to get to an onsen, it will probably be good. When you arrive and see the cliffside below the hot spring buildings covered in orange covered runoff from the baths, then you know it will be good.
You enter from the top and head down to the baths, but I am going to write this from the baths, or the bottom, up.
The baths are at the lowest floor of the building. There is a free small locker for wallets and other things if you need it as there are no lockers in the changing room. This is ryokan style with some baskets only. Also no toilets in the changing room. Use those before you head to the baths. There are two baths. A large indoor one and a smaller outdoor one. Both baths are encased in years of mineral buildup just like the hillside below the baths. The spring comes in via a large grey pipe then fills the tub in the front corner. That corner will be the hottest. This is kakenagashi straight from the well water at the 39°C temperature it comes out of the earth. The air in the room smelled oddly like kerosene, but that only enhanced the effect of being here. I thought they might have been heating the water, but I doubt that. The bath had a magical air to it. After washing off I slowly got in as the water is so opaque you cannot see in the baths. Be careful moving around in the baths here so you do not stub your toe. The outside bath is smaller and fed from the larger indoor bath. Both the men’s and women’s baths drain into a gutter between them and underneath the dividing wall. You can see all the spring water flowing out from the baths down the hill then it the damned Yoshino River. The soap and whatever else you wash off in the showers also just directly flows into the river. Everything here looks hand made and hodgepodge--making it even more of a great experience.
The water. Magical. Just magical. It is worth the trip and I wish I made it out here years ago.
On the next floor up there is a room to relax in with great big windows overlooking the river and mountains. The view here is amazing. If you look out the side windows to the right, you can see the source of the spring under a wooden shed. There are toilets and vending machines on this floor. Next to the stairs, the entrance to the source of the spring is hidden behind a plastic sheet door.
Up one more floor there are more toilets. Up to the floor you enter on, there is a restaurant and front desk--the stairs to the street above.
Access is difficult. On weekends and holidays the YuYu Bus can take you to the Osako Dam (大迫ダム) 4 km away. One bus leaving Yamato Kamiichi Station (大和上市) on the Kintetsu Yoshino Line at 0829 in the morning and arriving at 0936. The returning bus leaves dam at 1615 in the afternoon giving enough time for the 4km walk each way and a good soak. As I visited the hot spring on a Friday, I drove from Sakurai Station on the JR or Kintetsu lines using my Times Car Rental membership. It takes 1 hour to 1 hour and 30 min to get to the spring depending on traffic.