Either a first developer cartridge or a second developer cartridge is attachable to an image forming device. The second developer cartridge has capacity for accommodating developer larger than capacity for accommodating developer of the first developer cartridge. The control device determines whether the attached cartridge is the first developer cartridge or the second developer cartridge; determines that the replacement of the attached cartridge is needed upon comparison of a consumed amount of developer based on the image data that is inputted with a first threshold value when the first developer cartridge is attached; and determines that replacement of the attached cartridge is needed upon comparison of a detection value with a second threshold value when the second developer cartridge is attached. The detection value is obtained based on the detection information outputted by the detection device.