Utilizing only image information of an observation image and a reference image, a strain amount calculation unit calculates strain parameters. The reference image may be generated from design data or a different observation image. Calculated strain parameters are determined using strain formulation coefficients. A compensated image forming unit strains the reference image or the observation image by the strain amount based on the calculated strain parameters. The compensated image formed in the compensated image forming unit is supplied to an identification unit, which compares the compensated image with the reference image or the observation image, and determines positions where the difference is large to defects.