伊豆国
Kōzu-shima has been inhabited since at least the Japanese Paleolithic era, and archaeologists have found Jōmon period stone tools made from obsidian in Shizuoka and Yamanashi Prefectures. Under the Ritsuryō system of the early Nara period, the island was part of Suruga Province. It was transferred to Izu Province when Izu separated from Suruga in 680 AD. During the Heian period, obsidian was the primary export from the island.