Nakabusa Onsen is a historic mountain lodge located at an elevation of 1,462 meters within Chubu Sangaku National Park in Azumino City, Nagano Prefecture. Situated at the trailhead for Mt. Tsubakuro (2,763m) in the Northern Alps, this solitary inn was founded in 1821 by Mosuke Hyakuose and boasts over 200 years of tradition. The property contains an exceptional 29 registered hot spring sources, most with temperatures exceeding 90°C, making it one of Japan's most abundant hot spring locations.

The springs consist of two types: simple sulfur springs and simple springs, both with a pH of 8.8, making them alkaline high-temperature springs. Source temperatures range from 56°C to 97°C, and the baths are 100% natural flow-through (kakenagashi) with no added water, heating, circulation, coloration, or chlorine treatment. The facility offers 14 different types of baths, including large communal baths, open-air baths, private rental baths like the "Root Bath," and a seasonal outdoor hot spring pool. The water has a silky, slippery texture and is renowned for its skin-beautifying properties.

The most distinctive feature is the "geothermal steaming experience" using ground heat exceeding 100°C. Visitors can steam potatoes, eggs, oyaki dumplings, handmade sausages, and other foods in dedicated steaming facilities - an experience included in day-trip plans. The grounds also feature Meiji-era historical buildings and sites connected to mountaineer Walter Weston, registered as national cultural properties.

Accommodation options include the main Shosen-kaku building, lodges, and tent camping, catering to everyone from climbers to therapeutic bathing guests. Meals feature local mountain ingredients, particularly wild vegetables and river fish. As a base for climbing Mt. Tsubakuro, many hikers use the lodge for pre- or post-climb stays, making it a popular mountain resort combining hot springs and climbing. Operating seasonally from late April to late November, it's a member of both the Japan Secret Hot Springs Preservation Association and the Japan Source Spring Inn Preservation Association, continuing to preserve its traditions.