Located in Shimosuwa Town, Suwa District, Nagano Prefecture, 'Yusen House Koyu' is a day-trip hot spring facility situated near the Suwa Taisha Shimosha Akimiya shrine. Built in 1987 on the historic site of 'Wata no Yu' and 'Koyu,' bathhouses that had been beloved by travelers along the Nakasendo road since before the mid-Edo period. The source is the legendary 'Wata no Yu' spring, associated with the Suwa Shrine deity legend where the goddess Yasakatome no Kami placed cotton soaked with hot water for her cosmetic use, and a spring emerged from that spot. The bathhouse is particularly renowned for blessing visitors with fertility, which is the origin of the name 'Koyu' (child's bath). The water quality is simple hot spring (mildly alkaline, low osmotic pressure, high temperature) with a pH of 8.2, characterized by soft, skin-friendly water. The facility is the largest communal bathhouse in Shimosuwa Onsen, featuring indoor baths, an open-air bath, waterfall shower, and jet baths. A rest area and free WiFi are available, with a large parking lot. At just 300 yen for adults, open year-round from 5:30 AM to 10:00 PM, it is widely used by both locals and tourists.