Once the private spa of the Sanada Clan’s samurai retainers during the Edo period, this bathhouse is the first of nine public bathhouses at Shibu Onsen. The waters of these bathhouses are supplied by natural hot springs and have varying mineral compositions. Each bathhouse is known for a particular healing property and Hatsu Yu’s water is said to help gastrointestinal ailments. The hot spring water is around 42 degrees Celcius, the highest of the nine public baths. Locals have maintained these buildings for centuries since the hot springs were reportedly discovered by Kyoto monks in the 14th century. Only guests at Shibu Onsen’s ryokans (Japanese inns) can access these buildings with a key provided by their lodgings.