In a flare value calculation method according to an embodiment, an average optical intensity is calculated for each of mask patterns in a case where an exposure process is performed on a substrate using the mask patterns. Then, pattern correction amounts for the mask patterns corresponding to the average optical intensity and information about the dimensions of the mask patterns are calculated for each mask pattern. Then, post-correction mask patterns are prepared by performing pattern correction on each of the mask patterns using the pattern correction amount. Then, a flare value of an optical system of an exposure apparatus is calculated using a pattern average density of the post-correction mask patterns.