Optical image data of a mask is acquired. Reference image data associated with the optical images is created from design pattern data. Regional image data that includes pixel values denoted by multi-valued resolution based on importance level information of the patterns is created from region data including at least one portion of the patterns defined in the design pattern data. Defect determination is conducted on a pixel-by-pixel basis by comparing the optical image data with the reference image data, by means of either a plurality of threshold values determined by each pixel value within the regional image data or a plurality of defect determination methods. Image data of a section whose Mask Error Enhancement Factor (MEEF) is equal to or greater than a predetermined value is created from the region data including at least one portion of the patterns defined in the design pattern data.