Ishi-yu is one of three communal bathhouses (sotoyu) in Bessho Onsen, Ueda City, Nagano Prefecture, known as the secret bathing spot of the legendary samurai Sanada Yukimura. Bessho Onsen boasts a 1,400-year history as the oldest hot spring in Shinshu (Nagano). Legend has it that Yamato Takeru opened seven hot springs and named them Nanakuri-no-Yu (Seven Sufferings Relief Hot Springs). The surrounding area is called the Kamakura of Shinshu, dotted with cultural treasures including the octagonal three-story pagoda of Anraku-ji Temple. Ishi-yu uses Source No. 4, combining free-flowing natural spring water with circulation filtration. Source No. 4 was drilled in 1976 from a depth of 350 meters, boasting a source temperature of 52.5 degrees Celsius and an abundant flow of 1,300 liters per minute. The water quality is a simple sulfur spring (mildly alkaline), beloved as a beauty spring that leaves skin silky smooth. The bathhouse features an impressive stone exterior and carries the romantic legend of Sanada Yukimura secretly bathing here. The waters are effective for neuralgia, muscle pain, joint pain, sensitivity to cold, fatigue recovery, and chronic skin conditions.