Naka-ku (中区) is one of four wards of Okayama, Okayama Prefecture, Japan. The ward has an area of 51.24 km2 (19.78 sq mi) and a population of 138,949. The population density is 2,711 inhabitants per square kilometer (7,020/sq mi). The name means "Central Ward." The Shujitsu Junior College is located here