PURPOSE: Not to be eroded as taking higher input voltage so as to increase welding speed and improve productivity by replacing part of a silicon steel sheet right under a work coil with a ferrite core.

CONSTITUTION: An impeder 4A is constructed so that the length L part of a laminated silicon steel sheet 9 positioned at the upper side of an impeder axial core 8 right under a work coil 2 is replaced with a ferrite core 10. Thus, the silicon steel sheet 9 at the upper side of impeder axial core 8 is divided into 9a, 9b. Why the inserting position of ferrite core 10 is right under the work coil is that the magnetic flux generated from the work coil 2 is made to maximum right thereunder, further, the inserting position is the position corresponding to the open seam part of skelp 1 and the magnetic flux is directly acted on the impeder core material, therefore the position is replaced with the ferrite core 10. By this method, the input power, that is, input voltage can be made to higher.