Marukoma Onsen Ryokan, located on the northwest shore of Lake Shikotsu, is a historic hot spring inn founded in 1915 by Sasaki Hatsutaro. Situated within Shikotsu-Toya National Park, it is a member of the Japan Secret Hot Springs Protection Association. The most distinctive feature is the natural outdoor bath with foot-source hot spring water, one of only about 20 such springs in Japan. This open-air bath is separated from Lake Shikotsu by rocks and maintains the same water level as the lake, changing with seasonal fluctuations. The depth varies from approximately 30cm in spring to 150cm in autumn, offering unique bathing experiences throughout the year. Hot spring water at 54°C bubbles up from beneath bathers' feet, with temperature regulated by a sluice gate connected to the lake. Lake Shikotsu has been ranked first in Japan 20 times in the Ministry of Environment's water quality measurements, displaying the beautiful blue color known as Shikotsu Blue. The surrounding mountains including Mt. Eniwa (1,320m), Mt. Fuppushi, and Mt. Tarumae offer spectacular seasonal scenery. The ryokan operates without public infrastructure, generating its own power and using lake water, exemplifying harmonious coexistence with nature.