PURPOSE: To eliminate thermal strain, insensitivity and assure magnetic directivity for iron core by employing the structure that a coil formed utilizing laser light is inserted into iron core.

CONSTITUTION: An iron core 1 is previously provided with a groove hole 5 at the area near the pole surface. After the iron core 1 is caulked by the caulking 3, the material to be used as the shading coil 2 is bent in the form of letter U. The iron core is inserted from the one side of groove hole 5 and the end coming out of the other side of hole is also bent, so that the both ends 4a of coil 4 are matched. The matching area of both ends of coil 4a are welded by laser and thereby a ring-shaped shading coil 4 is formed. The welding can be realized without magnetic blowing out phenomenon since the laser light is converted to thermal energy and the light can also be squeezed. Accordingly, heating is very short and gives little influence on the iron core. Thereby, thermal strain, insensitivity and non-magnetic directivity for the iron core can be eliminated.