When a new developer cartridge is initially mounted in an image-forming device, a toothed part of a sensor gear disposed in the cartridge is brought into meshing engagement with an agitator drive gear disposed in the image-forming device. The sensor gear is driven while its toothed part is in meshing engagement with the agitator drive gear and driving of the sensor gear is stopped when its toothless part opposes the agitator drive gear. One or more contact protrusions are formed on the sensor gear to be movable therewith. An information-detecting mechanism detects how many contact protrusions are formed on the sensor gear during driving of the sensor gear. Based on the detection results, whether the mounted developer cartridge is a new product or not is determined, and information on the maximum sheets to be printed with the mounted developer cartridge is acquired.