Abstract: An electron gun is composed of a hemispherical cathode (1) and a second bias electrode (8) having apertures (9, 7, 11) along an optical axis of an electron beam fired from the electron gun, a first bias electrode (6) and an anode (10), arranged in that order, as well as a controller for variably controlling an electric potential applied to the first and second bias electrodes. The controller, for example, holds the sum of the electric potentials of the first and second bias electrodes relative to the cathode (1) substantially constant. Further, by adding one or more third bias electrode(s) (20) between the first and second bias electrodes (6, 8) as necessary, the intensity of the electron beam discharged from the high-intensity, high-emittance electron gun can be adjusted without affecting the current density angular distribution.