1. A lattice strain measurement method that quantizes lattice strain of crystal material constituting a measurement object, comprising the steps of: obtaining a HOLZ pattern by causing convergent beam electrons to impinge on the crystal material; transforming coordinates of a plurality of points extracted from the HOLZ pattern into Hough transform images by means of the Hough transform, extracting a plurality of clusters of Hough transform images, and specifying HOLZ lines by means of reverse transformation of the clusters; and quantizing the lattice constant of the crystal material in accordance with positions of the specified HOLZ lines.