100% source onsen called "Shyakunage no Yu" in Gunma prefecture, Numata city. If you are into wooded buildings and love the timeless feel of bathing in an onsen surrounded by nature then this is a good place to visit.
A time system is used: 500 yen for two hours, 800 yen for four hours, 1000 yen for 6 hours, and lastly 1200 yen for all day use. You get your moneys worth if you purchase the all day pass. This way you can take a nap, eat, and then go back. There's also a family bath available at 1000 yen for one hour, reservations only. Normal business hours are from 10 to 9 . Tel: 0278-20-0011. Having a car would be the best way to get here or you can get here via Takasaki Station. Call in advance for a shuttle bus.
Remember to observe the rules of onsen etiquette!