An electron gun comprising a cathode having an electron emitting surface and whose planar shape is circular; a heater; an anode being arranged to oppose the cathode; and a heat resistant member. The anode applies a positive potential relative to the cathode to extract electrons in a predetermined direction. The cathode has, in a central portion thereof, a through hole along a central axis of the cathode. The heat resistant member has a first portion to close the through hole and a second portion being positioned between the cathode and the heater.