cartridge c is installed into the image forming apparatus l the drum sensitivity detecting portion 60 within the control section of the main assembly reads out the sensitivity value in the memory 62 in this embodiment the drum sensitivity is divided into three ranges l m and h depending on the potential level vl of each photosensitive drum at the time of shipment the potential level ranges are h u2267 u2212 120 v m u2212 120 to u2212 170 v and l u2266 u2212 170 the charge and development dc voltages are varied according to each of the three drum sensitivity ranges with reference to the sensitivity conversion table 70 in the control portion 65 based on the relationship in fig1 the value of the unit step by which the development bias is varied is set to 20 v one unit step of change 20 v in consideration of the fact that the increase in the fog caused by the bias variation must be prevented it is necessary for both the charge bias and development bias to be varied by a predetermined unit of change so that back contrast bias and development bias to be varied by a predetermined unit of change so that back contrast and development contrast remain constant in this embodiment in consideration of the values max and mini of the maximum and minimum densities respectively which can be inputted by a user the unit step value by which the development and charge dc voltages are varied are set as follows development dc voltage variation unit u2212.