Ao-no-domon is a tunnel located at the foot of Mt. Kyoshuho on the Yamakuni River in Hon-yabakei-machi- Kiso, Nakatsu City, Oita Pref. About 250 years ago, the priest Zenkai, who dropped in at this place on his way traveling throughout the country, witnessed travelers and horses had difficulty or sometimes lost their lives in walking on a precarious path beside the steep cliff and decided to dig out a tunnel for safe traveling. He completed this tunnel with only a chisel and a hammer devoting as long as 30 years of his life. The tunnel has now a length of 342 m and has been improved for cars to run through. There traces of chiseling and a window for letting in the light still can be seen on the wall, from which we can infer the priest Zenkai’s strong will. His story is well known through Kan Kikuchi’s novel “Onshu no Kanata ni.” Later the toll was charged for passing the tunnel and it is said that this was the first toll road in Japan.