Aoki Kosen

Aoki Kosen stands alone deep in the mountains of Nirasaki City, Yamanashi Prefecture, at roughly 1,150 meters above sea level at the Dondoko-sawa trailhead for the Hoo Sanzan peaks of the Southern Alps. Founded in the early Meiji era, the current main building reproduces an 1870 structure, rebuilt in 1950 using traditional joinery without nails. Exposed ceiling beams, round windows, and snow-viewing shoji screens preserve the romanticism of the Meiji period. The celebrated Meiji essayist and poet Keigetsu Omachi is said to have stayed here regularly, and today the inn serves as a base for climbers heading into the Hoo Sanzan range.

The water is a ferruginous spring, historically classified under the old Japanese naming system as ryokubansen, a sulfate-type iron spring. Because the source emerges at only about 20 degrees Celsius, the inn heats the water before it reaches the baths. It is weakly acidic and odorless, faintly cloudy, and carries a pale brown sediment that rises to the surface when heated. Official figures list an evaporation residue of 271 mg per liter, with abundant sulfate and iron followed by aluminum, and the water is traditionally said to benefit neuralgia, digestive complaints, anemia, and heart conditions.

Bathing is limited to separate indoor baths for men and women. In contrast to the weathered exterior, the bathing rooms are spacious with plenty of washing stations. Day-trip bathing runs from 9:00 to 19:00 at 1,000 yen for adults and 850 yen for children. Lodging is room-only with no meals served, so guests bring their own food, from 7,700 yen per night. A campsite for up to 100 tents adjoins the inn. The season runs from late April to mid-November, with a winter closure from around November 16 until roughly April 27.