Yudanaka Oyu is a historic public bathhouse in Yudanaka Onsen, Yamanouchi Town, Nagano Prefecture. It holds the prestigious title of 'East Yokozuna' (east champion) in the Public Bathhouse Rankings published by the Japan Hot Spring Association magazine, alongside Dogo Onsen as the 'West Yokozuna'. The hot spring was discovered in the 7th century during the reign of Emperor Tenji (626-672) by the monk Chiyu, who named it 'Yokarei', meaning longevity and good health. The name signifies that this miraculous spring heals illness, refreshes the spirit, and promotes long, healthy life. During the Edo period, it flourished as a therapeutic retreat for the Matsushiro Domain. This is the largest public bath in Yudanaka Onsen, featuring an atmospheric wooden structure with high ceilings that create an open, spacious feeling. The bathing area is divided into two pools - one hot and one lukewarm - allowing bathers to choose their preferred temperature. The spring water is a sodium-chloride hot spring with a source temperature of 95.7 degrees Celsius and a mildly alkaline pH of 8.3. The flow rate is 100 liters per minute (naturally spouting from drilling). The water is rich in natural moisturizing metasilicic acid (254mg) and antibacterial metaboric acid (approximately 100mg). It is effective for neuralgia, skin conditions, poor circulation, cuts, and gynecological issues.