PURPOSE: To easily and accurately align an electron beam by irradiating the circularly polarized electron beam to sensing plates in an insulating state which are respectively disposed in the positions symmetrical with each other and detecting the current values flowing in the respective sensing plates.

CONSTITUTION: A center 31 of the beam is moved around a circle so as to cross four sheets of the sensing plates 22aW22d. The difference in the currents flowing in the plates 22a, 22c is compared by a comparator 42 and the extent of the movement corresponding to the current difference is applied to the X- direction command of a table so that the currents flowing through the plates 22a and 22c are made equal. The movement in the direction Y with respect to the difference in the currents flowing in the plates 22b and 22d is similarly executed so that the electron beam 10 is aligned to the center of a detector 21. The electron beam having a high energy density is thus easily and accurately aligned.