A flare map calculating method of an embodiment calculates an optical image intensity distribution in each division region set in a pattern region. Furthermore, an average value of the optical image intensity distribution is calculated in each division region. A pattern or plural patterns, which has a pattern density corresponding to the average value, is calculated as a corresponding density pattern in each division region. Furthermore, a density map, which represents a pattern density distribution within the pattern region, is generated based on the corresponding density pattern, and a flare map representing a flare intensity distribution within the pattern region is calculated by convolution integral of the density map and a point spread function.