A light traveling permissible portion and a light traveling blocking portion are alternately disposed in a specification detecting and agitator driving gear such that a detection portion detects information corresponding to the maximum image formation sheet number. When the developer cartridge is new, a new/used cartridge detecting gear restricts transmission of the aforementioned information from the specification detecting and agitator driving gear to the detection portion for a predetermined time t, thereby detecting whether the developer cartridge is new or used. The detection portion detects whether the developer cartridge is mounted based on whether the detection light reflects from the light traveling permissible portion.