PURPOSE: To prevent internal defects and to improve the quality of welding by controlling oscillation in an arbitrary constant direction with respect to the direction tangential to a weld line.

CONSTITUTION: An operator 19 receives a speed signal Vx in a direction x, a speed signal Vy in a direction Y, an oscillation direction signal α, and a basic wave generating signal f (A, wt) for oscillation as input signals and operates these signals so as to generate oscillation f(A, wt) wherein the oscillation direction with respect to the weld line is α, and outputs Ix, Iy. The Ix drives an X deflection coil 14 and Iy and Y deflection coil 15, and an electron beam 2 is subjected to synthesized deflection. In other words, the electron beam 2 performs the oscillation wherein the oscillation direction with respect to the weld line is α, the amplitude is A and the angular frequency is w.