To address the problem in which when measuring the overlay of patterns formed on upper and lower layers of a semiconductor pattern by comparing a reference image and measurement image obtained through imaging by an SEM, the contrast of the SEM image of the pattern of the lower layer is low relative to that of the SEM image of the pattern of the upper layer and alignment state verification is difficult even if the reference image and measurement image are superposed on the basis of measurement results, the present invention determines the amount of positional displacement of patterns of an object of overlay measurement from a reference image and measurement image obtained through imaging by an SEM, carries out differential processing on the reference image and measurement image, aligns the reference image and measurement image that have been subjected to differential processing on the basis of the positional displacement amount determined previously, expresses the gradation values of the aligned differential reference image and differential measurement image as brightnesses of colors that differ for each image, superposes the images, and displays the superposed images along with the determined positional displacement amount.