Located in Okutsu, Kagamino Town, Tomata District, Okayama Prefecture, Okutsuso is a historic hot spring inn founded in 1927 (Showa 2). Okutsu Onsen is counted among the three great hot springs of Okayama Prefecture (Yubara, Yunogo, and Okutsu), with a history stretching back to the early Edo period when the first lord of the Tsuyama domain, Mori Tadamasa, locked the bath to keep it for himself, giving rise to the name Kagiyu (Key Bath). The inn was registered as a National Tangible Cultural Property in November 2018.

The spring quality is alkaline simple hot spring with a pH of 9.1, known as a bijin-no-yu (beauty spring) for its skin-smoothing properties. The source temperature is 42.6 degrees Celsius, with 247 liters per minute gushing naturally from underfoot, delivered as completely free-flowing source water with no heating, dilution, or recirculation. Among approximately 35,000 hot spring facilities nationwide, only 20 to 30 offer this rare ashimoto yushutsu (underfoot spring) experience.

The inn features four baths: Kagiyu, Tachiyu, Izumi-no-Yu, and Kawa-no-Yu, all available 24 hours. Kagiyu utilizes riverbed granite with water welling up from underfoot, while Tachiyu is approximately 120 cm deep, allowing bathers to soak while standing. The world-renowned woodblock print artist Munakata Shiko frequently visited between 1947 and 1953, and his works are displayed throughout the inn. Day-trip bathing is available for 1,000 yen per adult from 10:45 to 14:30. The inn has 8 guest rooms including 2 detached rooms with private open-air baths.