Koakazawa Onsen Rakuyokan is a day-trip hot spring facility located in the secluded Akiyamago region of Sakae Village, Shimominouchi District, Nagano Prefecture. Situated in the center of Koakazawa settlement at the trailhead of Mount Naeba, it stands along the Nakatsu River valley at an elevation of approximately 600 meters. Akiyamago is a mountainous area spanning Tsunan Town in Niigata Prefecture and Sakae Village in Nagano Prefecture, historically known as one of Japan's heaviest snowfall regions, once isolated from the outside world during winter.

The main feature of this hot spring is its reddish-brown turbid water rich in iron. While the water emerges colorless and transparent at the source, it oxidizes upon contact with air and transforms into a reddish-brown color in the bathtub. The intensity of the color is described as "miso soup level," with such high mineral content that it stains towels. The water contains more than twice the general therapeutic spring standard, with total mineral content reaching 23,350mg/kg. Notably, it contains 32.66mg of ferrous ions (iron) and 813.8mg/kg of free carbon dioxide.

Rakuyokan was established as a village-run facility in 1984. The building features a mountain lodge-style wooden structure with a steep-angled roof reminiscent of gassho-zukuri architecture, reflecting its location in a heavy snowfall area. The bathing area has two pools of different sizes, equipped with reclining baths, pressure baths, and overhead baths. Hot water gushes from the spout like a geyser, and you can observe carbonic acid splashing about.

Mineral deposits form crispy films on the edges of the bathtubs, and breaking through these while bathing is one of the unique experiences. The temperature is moderate and set slightly warm, allowing for extended soaking. The spring water is 100% source-fed with no dilution, heating, or chlorine disinfection.

Also known as "Women's Bath," it is believed to be effective for anemia and menstrual disorders due to its iron content, addressing women's specific health concerns. The chloride spring provides excellent heat retention and moisturizing effects, warming the body from the core. The facility includes a dining area serving local cuisine and a second-floor rest area, beloved by mountain climbers and hot spring enthusiasts.