Ikeda Radium Kosen Hosenkaku is a historic therapeutic inn located in Sanbecho Ikeda, Oda City, Shimane Prefecture, at the foot of Mount Sanbe at an elevation of approximately 350 meters. Established in 1915 during the Taisho era, this secluded single-inn hot spring draws its mineral water from a former silver mine tunnel known as a mabu. In 1924, radon was confirmed in the water, and a subsequent survey in 1939 recorded an astonishing 6,460 Mache units of radon content in the No. 8 spring — declared the highest in the world at the time. This concentration is roughly nine times greater than Japan''s second-ranked Masutomi Radium Onsen in Yamanashi Prefecture.

The spring is classified as a radioactive sodium chloride spring (isotonic, neutral, cold mineral spring) with a source temperature of approximately 17 to 19 degrees Celsius and a pH of 6.0. The total dissolved mineral content reaches 11.22 g/kg, making it remarkably concentrated. The water has a distinctive bamboo-grass turbidity with salty and carbonated flavors, and calcium crystalline films from the mineral deposits can be observed floating on the bath surface. The water is served as free-flowing source water, with heated indoor baths and unheated cold source water outdoor baths available.

The facility retains the rustic atmosphere of a traditional therapeutic inn, featuring indoor baths for men and women and an outdoor bath. Visitors can also experience tunnel bathing, inhaling radon inside the former mine shaft. Day-trip bathing costs 500 yen for adults and 300 yen for children, available from 9:00 to 15:00. Overnight stays start at 7,500 yen per person with two meals. The spring is valued for its therapeutic effects on rheumatism, neuralgia, skin conditions, gynecological disorders, and cold sensitivity, attracting long-term therapeutic bathers.