The purpose of this paper is to clarify how a harbor city established in a sandbank inlet transformed a local area. In the Later Medieval period, the harbor city of Nakamura was established in the lower part of the Chikusa river. In the late 15th century, the Eiōji temple of Jōdo Shinshū Buddhism was constructed in the harbor city of Nakamura. Nakamura used Sagoshinoshō as a local name. In the 16th century, the harbor city of Kariya on the west coast of the Chikusa river also used the local name of Sagoshinoshō. In the early modern period, villages to the west of the mouth of the Chikusa river came to use the local name of Sagoshinoshō. While the local name of the west coastal area of the Chikusa river mouth had been Akōnoshō, it changed to Sagoshinoshō. Areas where local names may have changed also include Shikatanoshō and Mikuriyanoshō elsewhere in the Later Medieval period.