PURPOSE: To measure accurately length of a specimen, by defining a diffraction grating available for use as the length-measuring master by using a stable quantity, such as wave length of light.

CONSTITUTION: Photo-resist coating is spread over the top surface of an InP semiconductor substrate 1 respecting a crystal body and then a fringe pattern is formed by an interference light exposure method using a projected beam (wavelength: 3,250) of, for instance, He-Cd laser. Next, by anisotropic etching, the primary diffraction grating 2 is formed as the standard gauge 3 with distances 0.2μmW10μm on the top surface of the substrate 1. At this moment, the grating 2 is shaped into saw-tooth arrangement on the basis of the characteristic of the anisotropic etching. And, by installing the gauge 3 and the specimen 4 in such a way that the grating faces the specimen 4 or by calibrating the measuring system using the grating 2 prior to measurement of the specimen 4, electronic microscopic examination is made and accurate measurement of the length of specimen 4 is made on the based on contrast of the specimen 4 with the diffraction grating 2.