The image forming apparatus onto which a cartridge having a developing apparatus and memory medium is detachably mountable, a detector for detecting toner concentration, and a control unit for selecting an operational mode from a first operational mode and second operational mode, wherein a toner supply member starts to supply a toner to the developing apparatus on the basis of a value detected by the detector in the first operational mode, and on the basis of a cumulative value of image signals for printing in the second operational mode, wherein the memory medium stores information based on which the second operational mode is switched to the first operational mode, and the control unit works to switch the second operational mode in operation to the first operational mode on the basis of information related to the cumulative value of image signals and information stored in the memory medium. This structure allows the image forming apparatus to keep toner concentration in the cartridge at an adequate level.