Inspection equipment frequency transforms a mirror electron image with respect to position coordinates, calculates a value of a frequency plane origin or a value at a vicinity of the frequency plane origin as a first measurement value, and calculates a second measurement value by totalizing values of image intensities in a certain area, the image intensities having been obtained through normalization by the origin value or origin-vicinity value and the frequency transform. The inspection equipment automatically controls the voltage of a wafer holder based on the first and second measurement values.