The lobby of this ryokan in Japan is similar to "Infinity Castle" in the anime Demon Slayer (Kimetsu no Yaiba), including the demon Nakime who plays the lute. Ryokan Ookawaso is an inn and hot spring, commonly known as onsen in Japan, located in the Ashinomaki area of Aizuwakamatsu city, in Fukushima prefecture, Japan.

This Ookawaso ryokan is also the inspiration for the mangaka (Japanese comic writer), Koyoharu Gotouge, for the illustration of the popular manga Demon Slayer. In the manga, Muzan's main antagonist's headquarters is "Infinity Fortress" or "Infinity Castle", having a complex, multi-level, and dimly-lit design, and there is an area where the demon Nakime plays the lute, similar to Ryokan Ookawaso .