PURPOSE: To produce the electron beam which causes no temperature variation where the cathode can be replaced with those made of W and LaB6 by making the cathode switchable from the heating power source having the time constant voltage characteristic of W to the power source having the time constant current characteristic of LaB6.

CONSTITUTION: The cathode electrode heating power source 11 is separated into the power source 12 having the constant voltage characteristic and the power source 13 having the constant current characteristic, and when employing W for the cathode an exchange switch 14 is used for the power source 12 having the constant voltage characteristic to switch to conduct with the input wiring 15 and the output wiring 16 of the power source while when employing LaB6 for the cathode the exchange switch 14 is employed for the power source 14 having the constant current charactristic to switch to conduct with the input wiring 15 and the output wiring 16 of the power source. Consequently the temperature of the cathode is prevented resulting in a stable electron gun having the unvarying brightness.