PURPOSE: To enable the convergence angle of electron rays to be continuously varied without shifting the focus of an electron ray spot by correlating changes in the excitation of the irradiation lens system with changes in the excitation of the objective.

CONSTITUTION: Electron rays becoming incident upon the front magnetic field lens 9a of an objective are converged with a predetermined convergence angle (α) to form an electron ray spot on the surface of a sample 1. Then, converged electron rays 11 form a converged electron ray diffraction image on a fluorescent plate or a photosensitive material 12. Electron lenses 6, 7, 9 and 10 are controlled after an arithmetic unit 15 obtains the focal distance (f1) of a second condenser lens 7 and the focal distance (f2) of the front magnetic field lens of the objective after the convergence angle (α) and the aperture of a movable diaphragm 8 are set in a controller 13. Thus, the focal distance (f2) of the front magnetic field lens of the objective can be varied by varying the focal distance (f1) of the lens 7 by varying the current for controlling the lens 7. Consequently, it is possible to continuously vary the convergence angle of electron rays without shifting the focus of the electron ray spot.