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.