According to an aspect of the invention, there is provided a method comprising detecting a defect of a pattern formed on the photo mask, acquiring a pattern image of a first region on the photo mask, extracting a pattern contour from the pattern image to acquire pattern contour extracted data, producing first graphic data based on the pattern contour extracted data and a pixel size, acquiring pattern data including the first region and corresponding to a second region from design data, producing second graphic data from the pattern data, replacing the second graphic data with the first graphic data to produce third graphic data only in a region where the first graphic data is superimposed upon the second graphic data, producing transfer patterns of pattern shapes represented by the second and third graphic data, and comparing the transfer patterns to judge whether or not the defect needs to be corrected.