Takaragawa Onsen Osenkaku

470 tatami of riverside open-air baths, where everyone wears the inn's bathing robe — Japan's most spectacular mixed bathing at a bargain price, if you can live with an old building, day-trip crowds and summer horseflies

Osenkaku stands alone on the Takaragawa stream, and people come for one thing: four enormous open-air baths strung along the river, 470 tatami in all, filled with free-flowing water from four wells on site. Three are mixed; one, Maya-no-yu, is for women only. Since April 2019 everyone in the mixed baths must wear the inn's own bathing garment — a dress for women, a waist wrap for men — so first-timers find it far less daunting than it sounds, and after the day visitors leave at about 16:00 the evenings and early mornings belong to staying guests. Japanese reviewers rate it 4.22 across more than 1,000 reviews on one major site and 4.0 across 1,821 on another, with the onsen scoring highest and cleanliness and facilities lowest. The wooden Honkan of 1955 is the building to stay in — corner rooms especially — while the 1936 First Annex is small and basic. Dinner is mountain food, with fried amago trout the dish guests remember. The complaints are consistent: horseflies bite in summer, some pools run cool (one posted 26°C in December), walking between baths in a wet robe is cold in winter, rooms have stink bugs and creaky floors, and most guests and staff are now from overseas, which some Japanese guests like and others find hard. Standard plans with two meals start around ¥17,000–19,000 per person.