A developing cartridge removably set in a laser printer includes a drive gear that inputs a drive force thereto, a detection gear that irreversibly moves from a unused position where the detection gear does not engage with the drive gear, to a used position where the detection gear does not engage with the drive gear, through a drive force transmitting position where the detection gear engages with the drive gear, and a contact member formed on the detection gear. Provided in a main frame of the laser printer is an actuator that moves the detection gear from the unused position to the drive force transmitting position as the contact member contacts the actuator when the developing cartridge is set in the main frame of the laser printer.