In an image forming apparatus group including a plurality of kinds of image forming apparatuses having different process speeds, and a cartridge which can be commonly used in a plurality of kinds of image forming apparatuses, when there is provided a cartridge which can be detachably mountable to both a first image forming apparatus and a second image forming apparatus having different process speeds, n a control operation is carried out such that the difference between the peak-to-peak A.C. voltages in first and second first image forming apparatus main bodies varies with an increase in amount of used toners in the cartridge. Even when use of the cartridge is changed between the image forming apparatuses in the middle of being used, the image density level of an output image can be maintained nearly constant.