The Kojigoku Hot Spring is located on the way to Hobaru garden, just a short way from the Unzen hot spring area. Surrounded by mountains, this area has been used for hot spring bathing since the Edo era. The Kojigoku Spa opened in 1919 (but was renovated in 1993), and takes its water from the secret hot spring directly below the building. The water from the spring is milky white with a lightly sulfurous smell and is said to be good for the skin.