Onjuku no Yu

Onjuku no Yu is a day-use bathing facility facing the Pacific Ocean in Onjuku, Isumi District, Chiba Prefecture, about a 6-minute walk from JR Onjuku Station on the Sotobo Line. It draws hot spring water from roughly 800 meters underground and is a bathhouse popular with local residents and visitors to this seaside town.

The water is classified as a sodium-bicarbonate spring (hypotonic, weakly alkaline, cold mineral spring), with a distinctive blackish-brown color and a notably thick, viscous texture. The facility states on its official website that it does not dilute the spring water with additional water, serving it as free-flowing (kake-nagashi); however, whether the water is heated, recirculated, or chemically disinfected is not confirmed from publicly available information. The mineral-rich water is described as warming the body deeply and keeping it warm for a long time.

The bathing area includes functional baths such as a main soaking bath, jet-pressure bath, and walking bath, along with a Finnish-style sauna and a cold plunge bath. Admission is 1,000 yen for adults (700 yen for residents of the Sotobo area) and 300 yen for children. Opening hours vary by season: weekdays typically run 16:00-19:00 in winter and weekends/holidays 14:00-19:00, with the facility closed every Tuesday (open on national holidays). Visitors should check the official website for the latest schedule.