Located in the deepest part of Naruko Onsen area in Osaki City, Miyagi Prefecture, this is a secluded hot spring inn in the Onikobe Onsen district. Founded in 1940, this historic establishment has only 8 guest rooms, making it a true hidden gem. The Onikobe area is famous for its geysers, with steam rising from various spots throughout the region, creating a unique volcanic landscape.

The main attraction of Houunkaku is the natural waterfall bath called "Sennin-buro" (Hermit's Bath), where a natural waterfall pool serves as an open-air bath. The sight of hot spring water cascading down as a river is truly spectacular. Surrounded by forest in a tranquil environment, the indoor baths are gender-separated, while the outdoor baths and waterfall pool are mixed-gender, though bath towels and bathing wraps are permitted for modesty.

The waterfall hot spring is entirely dependent on nature, with bathing possible only from June to October when temperatures are suitable. After rainfall, the water temperature may drop too low for bathing, making this a genuine natural hot spring experience subject to weather conditions.

The inn has two source springs, with the indoor and mixed outdoor baths using the facility's own springs. Source temperatures range from 82-98°C, extremely hot, but instead of simply adding cold water, they cool the baths naturally overnight, maintaining 100% pure source water flow-through. Day-visit bathing hours are limited to 10 AM to 1 PM to allow sufficient time for cleaning and temperature adjustment.

Featured in Osaki City tourism posters showing sumo grand champion Hakuho bathing here, it enjoys high popularity within Miyagi Prefecture. The riverside open-air baths offer views of fresh greenery and autumn foliage, making this a precious hidden hot spring where visitors can simultaneously enjoy seasonal natural beauty and high-quality thermal waters. Hot spring enthusiasts visit from across the nation.