Prior to the execution of a printing operation, a print preparatory operation is conducted only on a priority developer cartridge out of the plurality of monochromatic developer cartridges mounted to the developing unit thereby bringing the cartridge into a printable state. Therefore, the print preparatory operation is completed when the print preparatory operation on one (equivalent to a "priority cartridge" of the invention) of the developer cartridges mounted to the developing unit is terminated. Hence, the apparatus may be placed in the printable state in a short time so that the print preparatory operation is prevented from compelling a user to wait needlessly long and that waiting time of a user may be shortened.