To prolong the service life of an electron gun by arranging a generating member which generates at least one of atoms forming a single crystal molecular structure near a cathode member formed of a single crystal of a molecule consisting of a specified atom.

A cathode member 13 is formed of a single crystal of LaB6, and a polycrystalline body 15 of LaB6 that is a generating member is arranged in the terminal part of the cathode member 13, and a graphite heater 12 is placed facing the cathode member 13 through the polycrystalline body 15. The surface of the body part 15 of a metallic aperture 14 is covered with a protecting film 17 consisting of a boride of a rare earth element having conductivity. When a negative charge is applied to the cathode member 13 having terminal part heated by the graphite heater 12 from the outside, thermoelectrons are released from the tip of the cathode member, and the polycrystalline body 15 is also overheated at this time to generate La atom or B atom. Accordingly, the La atom or B atom separated from the cathode member 13 is replenished with the La atom or B atom generated from the polycrystalline body 15.