When a developer cartridge is mounted together with a drum unit in a main body casing, a detection gear is rotated by driving of a motor, and an abutment projection of the detection gear is brought into abutment against a lever of a drum subunit to move the lever. The movement of the lever is detected by an optical sensor. A CPU judges information on the developer cartridge on the basis of the results of the detection by the optical sensor. Therefore, the information on the developer cartridge can be judged with a simple construction at reduced costs. Further, the developer cartridge can be mounted in the main body casing in spaced relation, so that the design flexibility is increased.