A highland hot spring located at approximately 1,000m elevation in Kokonoe Town, Kusu District, Oita Prefecture, within the Kuju mountain range. The hot spring was first discovered in the late Edo period on the slopes of Mount Hossho (1,450m elevation) as an acidic ferrous spring, making it the highest-altitude acidic spring in Kyushu. Currently operated as the Kuju Hossho Hotel, the facility boasts four different natural spring sources. These include a fumarolic spring (simple hot spring, 94.1°C, pH 6.8), a sulfur spring (simple sulfur cold mineral spring, 24.1°C, pH 5.2), a cold spring (simple sulfur cold mineral spring, 17°C, pH 4.6), and the Mountain Spring (acidic iron-sulfate-chloride spring, 47.9°C, pH 2.1). All four sources are provided as free-flowing kakenagashi. The Mountain Spring is particularly notable with its strong acidity of pH 2.1 and rich mineral content of 2,407.5mg/kg, and is one of the rare acidic springs that can be consumed as drinking water. The panoramic open-air bath Sankei-no-Yu offers magnificent views of the Kuju mountain range, with 16 different types of baths including rock baths, waterfall baths, cold spring rock baths, barrel baths, and jacuzzi baths. The natural beauty changes with the seasons: Miyama Kirishima azaleas in spring, fresh greenery in summer, autumn foliage, and snow-covered landscapes in winter. Day-trip bathing costs 1,000 yen for adults.