Method for generating data for an original plate used during processing for illuminating the original plate and projecting an image of a pattern onto the original plate onto a substrate via a projection optical system. A two-dimensional transmission cross coefficient is calculated based on a function indicating a distribution of an intensity of light formed on a pupil plane of the projection optical system. An approximate aerial image is calculated based on the calculated two-dimensional transmission cross coefficient and a first pattern on an object plane of the projection optical system. A second pattern is generated having the first pattern on the object place and auxiliary patterns based on the approximate aerial image. The original plate data is generated by repeatedly calculating the approximate aerial image and generating a second pattern that is used as the first pattern on the object plane.