Koyasu Onsen is a day-trip hot spring facility in the Shinshu Takayama Onsen Village in Takayama Village, Nagano Prefecture, which opened in 1991. The water quality is iodine-containing sodium-calcium chloride spring, classified as a rare iodine spring with only two such day-trip facilities in all of Nagano Prefecture, which has the most day-trip hot springs in Japan. It is certified as a therapeutic spring with expected medical effects due to its temperature and mineral quality and concentration. The source temperature is a mild 35.4°C and is heated for bathing, but it is 100% free-flowing without dilution or disinfection. The mineral content totals approximately 8,000mg per kg, a high concentration containing sodium, calcium, sulfate, iron, iodine, and many other substances. The high iron content causes the water to turn into a brown golden bath when heated and exposed to air. It has bactericidal, antibacterial, and keratin-removing properties, and is expected to be effective for skin conditions such as atopic dermatitis. Located near Koyasu Shrine, which enshrines the deity of safe childbirth, conception, and child-rearing, it is also known as the fertility hot spring.