To include generating a reference image based on a comparison between design data of a mask having patterns and an optical image of the mask in a first region of the mask designated in advance, and confirming whether the generated reference image has effectiveness, the generating and the confirming being performed by a reference circuit, in which the confirmation on whether the reference image has effectiveness includes adding, as a confirmation region in which whether the reference image has effectiveness is to be confirmed, a second region of the mask in addition to the first region set in advance as the confirmation region, the adding being performed by an addition circuit, and confirming whether the reference image has effectiveness in the confirmation region including the first region and the second region, the confirming being performed by the reference circuit.