1. A flare map calculating method comprising: dividing, by using a computer, a pattern region having a plurality of patterns to set a plurality of division regions in the pattern region; performing, by using a computer, a first exposure simulation in each of the division regions to calculate an optical image intensity distribution in each of the division regions; calculating, by using a computer, an average value of the optical image intensity distribution of the division regions as an average value of optical intensity distribution in each of the division regions; generating, by using a computer, a pattern or plural patterns, which has a pattern density corresponding to the average value of the optical intensity distribution, as a corresponding density pattern in each of the division regions; generating, by using a computer, a density map representing a pattern density distribution within the pattern region based on the corresponding density pattern; and calculating, by using a computer, a flare map representing a flare intensity distribution within the pattern region by convolution integral of the density map and a point spread function.