Furoukaku is a therapeutic hot spring inn situated at an elevation of approximately 1,050 meters within the Chichibu-Tama-Kai National Park in Sutama Town, Hokuto City, Yamanashi Prefecture. Nestled in a valley along the Hontani River at the foot of Mount Mizugaki and Mount Kinpu, its history dates back approximately 450 years to the Sengoku period, when the warlord Takeda Shingen used the Masutomi springs as a therapeutic bathing site for soldiers wounded in the Battle of Kawanakajima and miners from the Koshu gold mines. The springs became known as 'Kinsen-yu' around 1887 (Meiji 20) and gained fame as a radium hot spring when the therapeutic effects of radium became widely known during the Taisho era. The inn boasts five natural spring sources, all flowing as 100% natural radium springs without any dilution or recirculation. Its centerpiece is a magnificent natural rock bath carved beneath a massive boulder, where radium-rich spring water naturally bubbles up from the bottom of the bath. The water is classified as a radioactive, carbon dioxide, sodium-chloride spring with a pH of 6.3 and a source temperature of approximately 20 degrees Celsius. Boasting the highest radium content of any hot spring in Japan, the inn employs a Ministry of Health-certified hot spring utilization instructor who provides guidance on proper bathing techniques. A radium vapor inhalation room is also available, allowing guests to absorb radium through respiration for a comprehensive therapeutic experience.