Provided is a method of manufacturing imaging optical elements which cause a plural light beams to enter a deflection unit, and guide those beams to corresponding surfaces to be scanned, the imaging optical elements being arranged optically at the same position, having the same optical performance, the method including: measuring, with respect to the imaging optical elements having the same optical performance, the optical performance at each of a plurality of positions of the different light beam passing states; calculating a correction shape of an optical functional surface of the imaging optical element based on a deviation amount from a design value of the optical functional surface of the imaging optical element; performing correction processing on a shape of a mirror-finish insert of a mold for molding based on the correction shape of the optical functional surface; and performing molding by using the mirror-finish insert subjected to the correction processing.