九十九里浜
Isumi District was one of the ancient districts of Kazusa Province. The district was named after the Isumi River, which runs from the mountainous areas of Katsuura to the south of Kujūkuri Beach on the Pacific Ocean. The district is mentioned in Nara-period chronicles under various names, usually as . The "Nihon Shoki" records the area as , and the "Kojiki" as . In the ancient period the district bordered Mōda and Unakami districts to the west, and Habu and Nagara districts to the north. In the Heian period (794–1185) the district was divided into two shōen estates; one to the north, and one to the south. Minamoto no Yoritomo (1147–1199) drew soldiers from both shōen estates with the assistance of the local warlord Kazusa Hirotsune (?–1184). Yoritomo went on to found the Kamakura Shogunate. Manki Castle and Ōtaki Castle were built in this period. Kuniyoshi, once a village and now part of Isumi City, became a noted market town in this period, and a "rokusai-ichi", or market held six days of the month, is held to this day. By the end of the Sengoku period (1467–1573) the district had been variously controlled by the Toki, Hōjō, and Satomi clans. The Satomi ceded the area to Oda Nobunaga (1534–1582) in 1590.