After six years of hard work and eleven expeditions, I finally discovered the cave. It's an old mine tunnel on Mount Oni-ga-jō. I believe this is the "legendary Oni cave" mentioned in the local town history, "Ōe-chō Shi" (lit. "Ōe-chō Town Records").

Location: In the mountains of Mount Oni-ga-jō, Minamiyama, Ōe-chō Town, Fukuchiyama-shi City, Kyoto-fu Prefecture, Japan.