Katsurano-yu Maruei Ryokan is a secluded hot spring inn located at an altitude of 1,400-1,500 meters on the northeastern foothills of Mount Norikura in the Northern Alps, in Matsumoto City, Nagano Prefecture. This historic ryokan was established in 1938 (Showa 13) and is known as a small, family-run establishment with only 11 guest rooms.

Shirahone Onsen has a long history, with hot springs said to have been flowing since the Kamakura period. It is believed that the warlord Takeda Shingen used these hot springs for therapeutic purposes during the Sengoku period. The area gained national recognition when it appeared in Kaizan Nakazato's serialized novel "Dai-bosatsu Toge" (Great Bodhisattva Pass) during the Taisho era as the "Shirahone Chapter." The entire hot spring area is designated as a "National Health Resort Hot Spring" by the Ministry of the Environment, and the hot spring's fumarolic mounds and spherical limestone formations are designated as National Special Natural Monuments.

The main feature of Maruei Ryokan is its authentic cloudy white hot spring water flowing directly from the source. The water emerges clear and colorless initially, but as hydrogen sulfide and calcium components become fine particles upon contact with air, it gradually transforms into a beautiful milky white color over time. An ancient saying proclaims that "bathing for three days will prevent colds for three years," and the waters are believed to be effective for gastrointestinal ailments, gynecological issues, and neuralgia.

The outdoor bath is mixed-gender, but consideration is given to women who can wear bath towels while bathing, and the cloudy water provides privacy without concern for visibility. From the bathing area, guests can view the surrounding trees and the natural monument fumarolic mounds, fully experiencing the wild atmosphere of this hidden hot spring. The baths are available 24 hours a day, and there are also private baths available for those seeking more privacy.

The cuisine features warm country-style dishes centered on local mountain delicacies such as river fish (char and carp), wild vegetables, and wild boar meat. Guests can also enjoy unique local flavors such as rice porridge cooked in hot spring water. The inn has received high ratings in reviews for its rustic hot spring atmosphere and is particularly popular among travelers who prefer the hidden hot spring ambiance that Shirahone Onsen is known for.