Kogane Onsen Caldera Onsenkan

Caldera Onsenkan is a day-use bathhouse run by the village of Okura in Mogami District, Yamagata Prefecture, and it belongs to the Hijiori Onsen hot spring district, whose waters are said to have been discovered in the year 807. The Hijiori resort sits on the floor of the Hijiori caldera, a basin roughly two kilometres across that was formed by an eruption some ten thousand years ago. The bathhouse stands about 1.5 kilometres beyond the inn-lined main street, in the Kogane district along the Nigamizu River, and is easy to spot thanks to its timber main building and the octagonal bath pavilion beside it.

The bathing water comes from the Kogane Onsen cooperative source and is classified as a sodium chloride and bicarbonate spring, hypotonic, neutral and hot. Notices posted on site give a source temperature of 50.6 degrees Celsius and a pH of 6.2. Because the spring emerges hotter than bathing temperature, cold water is added, but the water is never reheated, recirculated or chlorinated, so it flows once through the tubs and away. The bath is a slightly cloudy green with brown mineral flakes drifting in it. A second spring on the same grounds rises naturally at just 9.4 degrees with a pH of 4.4, a naturally carbonated cold mineral water that feeds a drinking fountain and a shallow tub used for soaking hands and feet.

Bathing is split between separate indoor baths for men and women and a stone-walled open-air bath whose use alternates between the sexes by time slot. Inside there are lounge, rest and meditation rooms. Admission is 500 yen for adults from junior high school age, 200 yen for elementary school children and free for younger children. Opening hours run from 9:30 to 19:00 from April through November, with last entry at 18:30, and from 10:00 to 16:30 from December through March, with last entry at 16:00. The bathhouse closes every Tuesday and Wednesday. Since Hijiori Ideyukan ended its day bathing service in March 2025, this has been the public day-use bath serving the Hijiori area.