Japan's first private wood-fired sauna in an onigiri shape, made with cedar from Mogami Town and hinoki used for the benches. The onigiri shape was designed as the most efficient form for heat to gather in the center of the ceiling and warm the core body temperature. A genuine wood stove from Nagano Prefecture creates pleasant heat and the flickering of flames, and self löyly is also available. To encourage digital detox, there is intentionally no clock inside the sauna, allowing guests to enjoy a luxurious retreat free from time and the constraints of daily life (advance reservation required, swimwear required).

A cold plunge bath filled with flowing groundwater, set on the wood deck just outside Omusubi Sauna. Fresh groundwater nurtured by Yamagata's rich natural environment overflows continuously, gently yet sharply cooling and tightening a body that has been thoroughly warmed in the sauna. After the cold plunge, guests can relax in the outdoor air bath area with turf, and even on rainy days, enjoy soothing breezes and totonou in an infinity chair.