Tochinoki Onsen Oyama Ryokan

Oyama Ryokan stands beside the Shirakawa river system on the southern floor of the Aso caldera, in Kawayo, Minamiaso Village, Kumamoto Prefecture. The Ayugaeri Falls drop roughly 40 metres directly in front of the inn, and the old-growth forest of Mount Kitamuki covers the far bank. The spring is written with characters usually read Tochigi but is pronounced Tochinoki here, and its waters were first used some 300 years ago. It served as a healing retreat through the Edo period, and the inn's own website notes that Saigo Takamori and Helen Keller both visited. Oyama Ryokan itself has been in business for more than 200 years. Damaged in the 2016 Kumamoto earthquake, it stayed closed for five years before reopening in August 2021 after a full renovation.

The water rises naturally, without pumping. The posted analysis names it a sodium-magnesium-calcium sulfate and bicarbonate spring, hypotonic and neutral, and the water itself is clear and colourless. The Minamiaso Village Ryokan Association describes it as an abundant free-flowing spring. Listed benefits include rheumatism, neuralgia, skin complaints, and anaemia. Exact source temperature and pH figures have not been published.

There are two main bathhouses, one for men and one for women, each with an open-air tub facing the waterfall. Two private baths, one lined with hinoki cypress and one built of rock, are free for overnight guests. The inn has 21 rooms, some with their own open-air bath, and rates start at 15,400 yen per person with two meals. Day visitors pay 500 yen for adults and 250 yen for elementary-school children and under; a private family bath is 2,000 yen for 60 minutes (2,500 yen on holidays). Parking is free for 23 cars.