PURPOSE: To improve the quality of a welded portion by use of a simple structure by generating a plurality of electron beams concurrently in one electron gun so that preceding welding is conducted by one electron beam and a re-melting process is performed by the other beam.

CONSTITUTION: Two filament units 2 are fitted in a holder 1 of an electron gun and individual filaments 3 are faced to two small holes 4a provided on a control electrode 4 as electron emission holes, thus forming electric fields proceeding toward two small holes 5a of an anode 5. Thermal electrons emitted from the filaments 3 are individually accelerated by the anode 5, and in passing through small holes 5a two electron beams 6 are formed, one of which is used for preceding welding and the other for a re-melting process. Accordingly, the structure of the electron gun can be simplified, and welding can be performed at high quality with no failure on the welded portion even if the plate is thick.