Awanoyu is a renowned hot spring inn located at an elevation of 1,400 meters in the mountainous area at the eastern foot of Mount Norikura in the Northern Alps, representing the famous Shirahone Onsen. Established in 1912 (Meiji 45), this historic ryokan boasts over 113 years of tradition. Shirahone Onsen itself is said to have been flowing since the Kamakura period, making it a hot spring area with over 600 years of history. During the Warring States period, it was reportedly used by Takeda Shingen as a "hidden hot spring" for treating injured soldiers and miners.

The area gained national recognition in the Taisho era when novelist Kaizan Nakazato introduced it in his novel "Daibosatsu Toge" (Great Bodhisattva Pass), praising it as "magnificently colorful scenery." The poet Bokusui Wakayama also famously visited multiple times for hot spring therapy to treat his gastrointestinal ailments, leaving behind travel writings.

The main attraction of Awanoyu is its large mixed outdoor bath, spanning about 70 tatami mats (approximately 116 square meters), which boasts the largest volume of hot spring water in Shirahone Onsen. With an abundant flow of 1,730 liters per minute—one of the largest in Nagano Prefecture—the hot spring water flows continuously from the source. The water is originally colorless and transparent at the source, but upon contact with air, hydrogen sulfide and calcium components form fine particles, transforming it into a beautiful milky white color. The water mystically changes to various hues such as milky cream color or cobalt blue depending on the season, weather, temperature, humidity, and light.

The spring quality is sulfur-containing calcium-magnesium-hydrogen carbonate hot spring with a pH of 6.3 (neutral to weakly acidic). Unlike typical milky white hot springs that are strongly acidic, Shirahone Onsen is gentle on the skin, making it suitable for everyone. Rich in carbonic acid gas, the lukewarm water at around 38°C causes fine bubbles to cling to the body, which is the origin of the name "Awanoyu" (Bubble Bath). These bubbles provide unexpected bathing benefits, giving rise to the legend that "bathing for three days will keep you from catching a cold for three years."

The indoor bath is made entirely of cypress, preserving the atmosphere of a traditional hot spring therapy site, with two baths: a 37°C lukewarm bath using pure source water and a 40°C heated bath. Separate outdoor baths for men and women are also available. The water is drinkable and is said to be effective for gastrointestinal diseases and diabetes. The specialty "onsen-gayu" (hot spring rice porridge) cooked with hot spring water is a breakfast delicacy.

Guests can enjoy the spectacular scenery of each season while bathing in this hidden gem among hidden hot springs: cherry blossoms and fresh greenery in spring, lush green mountains in summer, brilliant autumn foliage, and snow-viewing baths in winter. Located within a national park, it is also designated as a National Health Hot Spring Resort by the Ministry of the Environment.