The Mikawa area has been inhabited since prehistoric times, as attested by finds of pottery shards from the Jōmon period and the megalithic Kofun tomb in Kira, the oldest in the Mikawa Province. The fertile plains along the Yahagi River have been used for rice-farming as well as the production of tea and cotton since ancient times. Shell mounds dating to the late Japanese Paleolithic period which have been found in what is today the town center also point to fish and seafood as important early local produce. In Hazu, a shrine from the Nara period is evidence of an early cultural connection to the Japanese capital at the time. The soil around Nishio is rich in mica deposits, which was already mined in the Nara period.