Kannojigoku Onsen is a secluded hot spring located at an altitude of 1,100 meters on the foothills of the Kuju mountain range in Kokonoe-machi, Kusu-gun, Oita Prefecture. This historic hot spring was first discovered in 1849 during the late Edo period, and Kannojigoku Ryokan was established in 1928. Legend has it that the hot spring was discovered when a hunter saw an injured monkey healing its wounds by bathing in the cold spring water.

The most distinctive feature of this hot spring is its rare natural cold spring water with a source temperature of 13-14°C. It boasts an abundant flow rate of 2,160 liters (approximately 2 tons) per minute and operates as a completely natural flowing spring. Known as one of the "Kuju Nine Hot Springs," it is also included in the "Kujukyu Mountain Range Hot Spring Resort" promoted by the Kuju-Iida Plateau Tourism Association. The traditional bathing method involves alternating between the cold spring and an adjacent heated room, known as "hot-cold alternating bathing." In ancient times, bathers would sing the "Cold Spring March" while bathing. In July 2023, the "Atsu-no-Jigoku Sauna" was newly established, making it possible to enjoy the cold spring bathing year-round, which was previously limited to summer months.

The main baths include the Kiriishi-yu (cut stone bath) and Hinoki-yu (cypress bath), which are wood-fired and operate on a daily alternating system for men and women. There are also three types of semi-open-air family baths available free of charge without reservation. The building maintains the atmosphere of an early Showa period hot spring resort with wooden architecture, and the dining area features an irori (traditional hearth) where guests can enjoy local mountain and river delicacies including Bungo beef. As a member inn of the Japan Secret Hot Springs Association, it continues to carefully preserve traditional hot spring culture.