Gold mines and Shingen
The name comes from Masutomi village, which existed from 1875 to 1959. Legend says Takeda Shingen found the spring while mining gold here (Wikipedia). Known as a therapeutic retreat in a mountain valley, it drew the haiku poet Takahama Kyoshi in 1917 and later the writers Ibuse Masuji and Tanaka Fuji. On 5 August 1965 it was designated a National Health Resort. It lies within Chichibu-Tama-Kai National Park, with Mt Mizugaki to the north and the Hontani River in the valley.[5][7]
Measuring radon
Scientific surveys at Masutomi go back to 1914. The Yamanashi Institute for Public Health records that Japan's highest radon concentration in spring water was observed at the area's A9 spring, and in 2017 and 2020 it examined 25 unused springs on the right bank of the Hontani. The therapeutic thresholds are 111 Bq/kg for a weakly radioactive spring and 673 Bq/kg for a radioactive one; survey point No. 2 gave 11,500 Bq/kg. Many of these springs were buried by road works and none is used for bathing. The inns' own sources are analysed regularly, but those figures were not readable on official sites.[3]
Long soaks in unheated water
Masutomi's sources are cool, around 30 °C; Furokaku posts about 30–36 °C in its indoor baths and 17–20 °C at the outdoor source. The inn describes its five sources as '100% natural, free-flowing' and grades the baths by how strongly they act on the body: medium, strong, strongest. Wikipedia gives bathing times of 15–30 minutes. This is the reverse of the usual Japanese quick hot dip: the idea is to sit still in water below body temperature and let the minerals work. Day visitors have only Furokaku's 12:30–14:00 slot, as the municipal Masutomi-no-yu has been closed since April 2023.[2][5][4]
