1. A method of mechanically measuring the density of a plurality of defects that occur in a single crystal for each type of defect, comprising: etching an observation surface, which is a surface of the single crystal, to form an etch pit at each defect; calculating, using a microscope, the maximum depth, mean depth and depth curvature of each of etch pits formed at a plurality of defects present within a predetermined area on the observation surface; and comparing the measured maximum depth, mean depth and depth curvature with respective reference values to determine the type of each defect within the predetermined area, and calculating the position of the center of gravity of a planar shape of each of the etch pits within the predetermined area; and calculating the maximum depth position of each of the etch pits formed within the predetermined area, wherein the type of each defect within the predetermined area is determined on the basis of an orientation of the measured maximum depth position with respect to the calculated position of the center of gravity of the planar shape of each etch pit.