An electron gun includes a plate-like main cathode 77 having an electron emitting surface 79 and a sub-cathode 81 provided toward the rear surface of the main cathode to heat the main cathode 77 by imparting an electron bombardment. The sub-cathode 81 is constituted of filaments 83 and 85 coiled in a double helix structure and the diameter of the sub-cathode 81 is larger than the diameter of the main cathode 77. As a result, the temperature at the peripheral area of the electron emitting surface 79 can be set higher than the temperature at the center, to achieve an electron beam with a uniform intensity distribution.