1. A computer readable storage medium storing a program for causing a computer to generate reticle data for producing a reticle used in an exposure apparatus in which an illumination optical system illuminates a reticle and a projection optical system projects a pattern of the reticle onto a substrate, the program causing the computer to execute processing including the steps of: classifying target patterns to be formed on a substrate into a plurality of direction groups; extracting, for each of the plurality of direction groups, a region suited to resolution of a target pattern belonging to the direction group from an effective light source distribution formed on a pupil of the projection optical system by the illumination optical system, thereby determining the extracted region as a partial light source; executing, for each of a plurality of partial light sources, processing of determining a pattern to be placed on a reticle when each partial light source is used as an illumination condition; and merging patterns determined in the step of executing processing.