Andai Ooyu is a communal bathhouse located in Andai Onsen within the Yudanaka Shibu Onsen area of Yamanouchi Town, Nagano Prefecture. Andai Onsen dates back to 1705 when a bathhouse was established on the riverbank of the Yokoyu River, and has flourished as a therapeutic bathing destination for about 300 years. Andai Ooyu is considered the origin of the hot spring, with local records stating that villagers came together to build a single bathing facility. The hot spring features sodium chloride-sulfate water with a mildly alkaline pH of 8.3 and a source temperature of about 93 degrees Celsius, offered as 100% free-flowing natural hot spring water without heating, circulation, or disinfection. The colorless and transparent water has a distinctive kelp-like aroma and leaves the skin feeling smooth and oily when touched while wet. The building is a substantial wooden structure with calligraphy by former Nagano Governor Goro Yoshimura displayed under the eaves. It has a unique structure with two entrances on opposite sides of a sloped street, and the tiled bathroom features blue round tiles and red-tiled cylindrical pillars. The large communal bath can accommodate about 10 people and is exclusively for local residents and guests staying at Andai Onsen accommodations.