A casing of a developing apparatus defines a developing chamber and a developer accommodating chamber, that accommodates developer, therein. The developing chamber and the developer accommodating chamber are arranged side by side. The casing is formed with a replenishing port for supplying new developer from a developer replenishing unit into the developing chamber. The replenishing port is located right above a supply roller when the developing apparatus is disposed in an orientation in which the developing apparatus is intended to be placed in a main casing of an image forming apparatus. The control unit controls the supply roller to rotate when the developing apparatus is mounted in the main casing. The supply roller stirs old developer that have already been located in the developing chamber and the new developer, and transports the old developer and the new developer from the developing chamber into the developer accommodating chamber.