An object of the present invention is to accurately verify errors, etc., in programs when corrections of mask data are carried out by the programs. In order to correct the mask data based on predetermined conditions, a method for mask data verification according to the present invention comprises the steps of preparing corrected mask data by using a plurality of programs each of which has a different algorithm, comparing each of corrected mask data which is prepared in the previous step and as a result of the comparison, if there are differences among the corrected mask data, extracting errors which cause problems as mask data from the differences.