Koyubo Gensenkan

Gensenkan is a therapeutic-bathing inn in Shimobe Onsen, a hot spring village strung along the Shimobe River in Minobu Town, Yamanashi Prefecture. Shimobe Onsen is said to have been opened some 1,300 years ago in the Nara period, and it is famous as one of the hidden healing springs of the warlord Takeda Shingen, who is believed to have treated his battle wounds here in the sixteenth century. The inn has been in business for roughly 250 years since the Edo period and bears the title Koyubo, which only the oldest inn in a hot spring village may use.

Its centerpiece is the Oiwa-buro, the great rock bath of Shingen's hidden spring. Water rises directly through cracks in the bedrock beneath the bathers at a rate of 200 to 400 liters per minute, making this one of the few genuine ashimoto-yushutsu baths in Japan, where the source emerges inside the tub itself. The water is a simple spring, weakly alkaline, at about 30 degrees Celsius with a pH of around 8.4. Nothing is added, heated, recirculated or chlorinated, and the Pure Onsen Association certifies it in its highest grade. The gender-separated baths in the main building draw a second source, the 49-degree Shimobe Oku-no-yu high-temperature spring at pH 9.4, and guests traditionally alternate between the cool and hot waters as a cure.

The Oiwa-buro is mixed bathing with separate changing rooms; bathing wear or a wrapped towel is required, and there are women-only hours. Bathing is available from 6:00 to 22:00. The inn has 24 rooms across a three-story wooden main building and the annex Shinsen, which has an elevator and is aimed at longer therapeutic stays. Day-trip bathing is not offered, so the baths are for overnight guests only. Rates with two meals start at 12,100 yen in the annex and 16,500 yen in the main building, plus a 150 yen bathing tax, with free parking for 15 cars.