Guide
Located along Prefectural Road 300 (Ujishimahara Line) in the mountainous region of Takahashi City, Okayama, the "Hayama Dai-ni Tunnel" is an extraordinary structure where natural geological formations and man-made engineering merge. The most striking feature is that the tunnel utilizes a natural limestone cave. Standing at the northwest entrance, you are greeted by towering limestone cliffs that create the atmosphere of a natural cavern entrance. It is considered a rare "unlined" (raw-cut) tunnel—meaning the original rock surface remains intact—making it a precious historical and geological site within Okayama Prefecture.
The history of this tunnel dates back to the Taisho era, with construction beginning in 1919 and completion in 1921. At the time of its opening, it played a vital role in transporting local goods. Historically, the "Fukiya Orai" route connecting Fukiya and Seiba ran along the ridge, transporting goods such as red ochre (bengala), copper, iron, and seafood from Kasaoka. To create a more efficient transport route, the current path along the Kikawa River was excavated. Due to this history of heavy transport, the tunnel was designed with a width of 3 meters to allow horse-drawn carriages and carts to pass through.
There are several notable features of the Hayama Dai-ni Tunnel. First is the northwest entrance, where the vertical limestone walls of the karst topography create a dramatic, cave-like entrance. Second is the "unlined" interior structure. Spanning 32 meters in length and 3 meters in width, the interior lacks modern concrete lining, leaving the raw, natural rock face exposed. The northwest cliffs are so steep and dynamic that they are occasionally used as a site for rock climbing practice.
As the tunnel is part of a public road (Prefectural Road 300), it is accessible at any time of year. Daytime is the best time to visit to appreciate the intricate details of the limestone texture and the surrounding scenery of Hayama Valley. However, please note that the tunnel interior and nearby crevices (such as the "Anagoya" or Seiryu Cave) lack lighting; exploration during the night or in dark conditions can be difficult due to low visibility.
The primary experience is enjoying the view during a drive or stopping for photography. The entrance area offers a photogenic view of the natural rock formations. For outdoor enthusiasts, the surrounding cliffs are known as a spot for rock climbing practice. Please note that because the tunnel is narrow, drivers should proceed with caution when passing through.
When visiting the Hayama Dai-ni Tunnel, we recommend combining it with a drive through the scenic Hayama Valley toward the Tenryu Bridge to the west. Near the bridge, you can find the "Kata Domon," where rock overhangs create a natural roof over the road, and the Hayama Dai-ichi Tunnel, which has a more modern concrete-reinforced entrance. This area is part of a larger scenic driving route, located approximately 30km from the Niimi IC on the Chugoku Expressway.
Since this is a public road, no special entrance fees or reservations are required. However, as it is a natural environment, comfortable walking shoes and clothing are recommended. Be aware that the road surface may be damp or dark depending on the weather and time of day. When driving, please maintain a safe speed and stay alert, as the road can be narrow.