PURPOSE: To adjust an irradiation position accurately in a very short time, by determining automatically the bias supplied to the deflection system of the electron beam when the electron beam is irradiated to the needlelike linear material difficult to set the welding position.

CONSTITUTION: For irradiation of the faint electron beam generated from electron gun 2, the signal obtained by adding 21 the bias signal of bias circuit 22 to the output signal of scanning signal generating circuit 12 is amplified in 23 and is supplied to X deflecting means 4. As the result, the electron beam scans a fixed scanning width from start point A of scanning to end point B across needlelike linear material 6. When the electron beam scans material 6, the signal is detected by 7, and further, this signal is amplified and is supplied to rise checking circuit 10 and fall detecting circuit 9 to generate a prescribed trigger signal. After that, scanning is so performed through prescribed operations that material 6 may be positioned in the center of the scanning range of the electron beam, and simultaneously, the intensity of the electron beam is enhanced to weld material 6.