Masutomi Radium Onsen Oyumoto Tsuganero

Tsuganero stands at around 1,000 metres above sea level along the Hontani River in Obi, Sutama, Hokuto City, Yamanashi Prefecture, on the edge of Chichibu-Tama-Kai National Park. The inn was founded in 1885 by Tsugane Taneshige in the Tsugane district of Sutama and moved to Masutomi Onsen in 1914. Local tradition holds that the Masutomi springs were found by the warlord Takeda Shingen while prospecting for gold veins, and that wounded soldiers of his army came here to recover. Surveys by Professor Kenjiro Kimura of Tokyo Imperial University in 1937 and 1938 confirmed the unusually high radon content, and the inn has since called itself the oyumoto, the principal source of Masutomi.

The water is a sodium chloride spring with a pH of about 6.4. It emerges from two springs that rise close to the surface at 26 to 30 degrees Celsius, cool enough to shift with the outside air temperature. No water is ever added, and the baths run on a full flow-through system with nothing recirculated. The main bath is arranged as two tubs: one filled with the untreated spring at source temperature, the other warmed to around 42 degrees by a steam boiler because the source itself is too cool. Bathers alternate between the two. The unheated tub carries a golden tint, the warmed one a faint dark cast.

There are two bathing areas: the main bath on the second floor, separated by gender, and Nyu no Yu on the fourth floor of the old wing, a family bath reserved for overnight guests that sits directly above the spring and reopened after renovation on 11 June 2026. The inn has 35 rooms for up to 120 guests, an 80-person banquet hall and parking for 30 cars. Rates start at 11,700 yen with two meals or 6,200 yen without meals. Day-use bathing is currently suspended, so the baths are open to staying guests only, and payment is cash only.