PURPOSE: To stabilize beam density distribution by arranging the number of Wehnelt beam permeation holes so as to obtain the same distribution as the beam density distribution in the cross-sectional direction of a desired composite electron beam.

CONSTITUTION: The size of beam permeation holes 3' is all the same and the distribution for the number of holes match the desired beam density distribution. For example, three holes are arranged at the center and respective five holes are arranged at upper and lower ends to obtain the double peak characteristics of beam distribution. In an electron gun with such structure, the relactive distance between a cathode and a Wehnelt cathode are constant and good in the beam cross-sectional direction. Consequently, the beam with uniform characteristics is emitted from each beam permeation hole and each beam is synthesized and the desired double peak characteristics are obtained. Since the relative distance between the cathode 2 and the Wehnelt cathode 3 are constant in the beam cross-sectional direction, the beam orbit length of each beam is constant. As a result, even when the distance up to the beam irradiated substance is varied slightly by deflection scanning and so on, the beam density distribution is not varied because the whole composite beam is even and convergent.