To prevent deterioration in the work function of an electron gun chip due to hydrogen ion bombardment and to repair the deformed shape of the chip (field emission cold cathode) efficiently.

The electron gun tip 3 covered with a film of materials resistant to hydrogen ion bombardment such as aluminum or carbon, etc., is spot-welded to a tungsten filament 4 and brazed with gold. Lead-out anodes 5 are made of main materials hardly absorbs hydrogen such as Al2O3, etc., coated with materials hardly absorbs hydrogen such as gold, TiN, AiN, etc. A digital controller 11 for repairing the chip digitally control the application of heating current pulse from a heating current pulse adjusting device 10 and reverse charge voltage from a reverse charge voltage adjusting device 9 to flash the electron gun chip 3 under heating and repair the shape.