1. A method for generating data for an original plate used during processing for illuminating the original plate with illumination light and projecting an image of a pattern on the original plate onto a substrate via a projection optical system, the method comprising steps of: calculating a two-dimensional transmission cross coefficient based on a function indicating a distribution of an intensity of light formed on a pupil plane of the projection optical system with the illumination light and a pupil function for the projection optical system; calculating an approximate aerial image obtained by approximating an aerial image on an image plane of the projection optical system by at least one component of a plurality of components of the aerial image based on the calculated two-dimensional transmission cross coefficient and a first pattern on an object plane of the projection optical system; generating a second pattern having the first pattern on the object plane and auxiliary patterns based on the approximate aerial image; and generating, by a control unit, original plate data including the second pattern generated by repeatedly performing the step of calculating an approximate aerial image and the step of generating a second pattern by using the second pattern generated as the first pattern on the object plane.