1. A mask evaluating method comprising: calculating a killer defect rate function with respect to a simulative defect pattern including a pattern of photomask and a plurality of defects, the killer defect rate function representing a killer defect rate of the simulative defect pattern with respect to a desired density of the pattern and a desired size of the plurality of defects; calculating the number of killer defects in an inspection region of the photomask based on an area of the photomask inspection region, a pattern density in the inspection region, the killer defect rate function and a defect size distribution representing the number of defects per unit area to a defect size range acquired from a photomask manufacturing step; and evaluating the photomask based on the calculated number of the killer defects.