To provide an electron gun and an electron-optical lens barrel, capable of reducing temperature rise of an electrode, resistant to the growth of contamination, and resistant to the occurrence of dielectric breakdown, etc.

A high-tension insulator 27 is fixed in an electron-gun chamber 24 of the electron gun 1. The insulator 27 is made of highly heat-conductive SiC or AlN. A Wehnelt electrode 22 and a control anode electrode 23 are mounted on an under surface of the insulator 27. A cathode 21 is fixed to a lower end of the electrode 22 by means of a quartz spacer 25. Since the spacer 25 increases thermal resistance between the cathode 21 and the electrode 22, a loss of heat escaping from the cathode 21 to the exterior is reduced. Since heat conduction via the insulator 27 is good, thermal resistance between the electrode 22 and the chamber 24 is reduced and temperature rise of the electrode 22 is suppressed, and therefore thermal expansion, an increase in discharged gas, dielectric breakdown, and the like are less liable to occur.