Tsunan is located at the very south of Niigata prefecture on the border between Nagano prefecture. Naeba mountain on the south and Shinano river runs through north to south, magnificent river terrace of nine tiers at most is formed along with Shikumi river, Nakatsu river and Kiyotsu river. Despite the heavy snow in winter people can enjoy cool and dry summer. These topography and weather make it possible to grow rice, flowers and plants, carrots, asparagus, sweet corns. There are lots of ruins from Jomon era in the town which proves that ancestors developed culture and life by utilizing river terrace.
Okinohara ruins dates back to the midst of the Jomon era and located on river terrace along the Nakatsu river, about 160 meter from the bottom of the river. Three excavation surveys were conducted between 1972 and 1973. It is estimated that the village, which was designated as national historic site in 1978, was in circle of 120 meter in diameter with a central square and inhabited more than 200 households. In addition to Kaen-gata earthenware vessels, stone tools, arms, and cookies were discovered and designated as tangible cultural asset by Niigata prefecture and preserved and presented in the Tsunan history and folklore museum.