PURPOSE: To provide a titled device which enables exact electron beam welding, by installing magnetism detectors on both sides of the weld zone in an advance direction of an electron gun and controlling the position of the electron gun according to the difference in the detected quantity of magnetism of objects to be welded.

CONSTITUTION: Magnetism detectors 14, 15 are installed on both sides of a weld line 13 where an objects 12 to be welded advancing in an X-axis direction are welded by the electron beam 11 emitted from an electron gun 10. The magnetism detection signals by these detectors are inputted through amplifiers 16, 17 to a comparator 18, by which the signals are added and a difference between both signals is drawn out. The differential signal is inputted through an amplifier 19 and a power circuit 20 to a driving device 21. The device 21 moves the gun 10 in a Y-axis direction according to the differential input signal and controls the position of the gun 10 in such a way that the beam 11 deflected by the difference in the quantity of charged magnetism on both sides of the line 13 is exactly located on the line 13.