The 239 m-long Sasago Tunnel was bored beneath Sasago Pass in 1938 to make the most dangerous section on the old Koshu-kaido Road safer. Until 1958, when New Sasago Tunnel opened, the old Koshu-kaido Road contributed to the economic growth and modernization of transportation in Yamanashi as the main road to Tokyo.
The tunnel has a reinforced concrete structure, with two decorative posts placed on the both sides of the entrance on the Otsuki Side, characterized by Western-style designs, and was registered as a national cultural property in 1997.