Prior to execution of a printing operation, a print preparatory operation is conducted collectively on all developer cartridges mounted to a developing unit, thereby bringing all the developer cartridges into a printable state. Therefore, even if a residual amount of toner in the cartridge runs too low to continue the printing operation performed using one of plural cartridges mounted to the developing unit, the printing operation is allowed to continue by switching the cartridge to the next cartridge. Hence, a large volume of monochromatic prints may be produced in an efficient manner.