The image forming apparatus has an image supporting body, a developing device on which developing units storing developers of the same color are detachably mounted, and control unit for controlling a printing operation in response to a print request. Further, in the case where the total number of print pages of a first developing unit, which is in use, exceeds the number of calibration pages if executing a print request, when a second developing unit which does not exceed the number of calibration pages exists, the control unit executes the printing operation using the second developing unit, and when the second developing unit does not exist, the control unit executes the printing operation using the first developing unit if the requested print page number does not exceed a predetermined threshold, or executes a calibration operation if the requested print page number exceeds the predetermined threshold.