Aizankei Onsen Aizankei Club

Aizankei Onsen is a single-inn hot spring at the foot of Mount Nagayama in Daisetsuzan National Park, in Kamikawa Town, Hokkaido. It sits at roughly 1,000 metres above sea level, surrounded by primeval forests of Ezo spruce and Sakhalin fir, some 19 kilometres up the Aizankei valley from National Route 39. The spring is said to have been found in 1909 by Tojiro Naoi of Aibetsu Village; it opened as Naoi Onsen in 1926 and was renamed Aizankei Onsen in 1936. In 2026, its centenary year, the non-profit organisation Kamui took over operations under contract with Kamikawa Town.

The water is a sodium-magnesium bicarbonate-sulphate spring, classified as hypotonic, neutral and hot. The source emerges at 44.2 degrees Celsius with a pH of 6.3, and is served entirely undiluted, unheated and unchlorinated as 100 percent free-flowing spring water. The operator describes it as a dense, iron-bearing water that clouds to a brownish tone. It is traditionally recommended for neuralgia, muscle and joint pain, fatigue, poor circulation, cuts, burns and arteriosclerosis.

The bathing arrangement is deliberately plain: one indoor bath each for men and women. Day-use bathing runs from 10:00 to 19:00, with last entry at 18:30, at 800 yen for adults and 500 yen for children. Guests may stay in the main eco-lodge, at 13,000 yen per adult with two meals, or in the self-catering Aizankei Hutte annexe at 4,000 yen per adult. The lodge runs on its own micro-hydro generator fed by the adjacent river. The season runs from May to October each year; the access road closes in winter and the inn shuts down.