PURPOSE: To enhance the demagnetizing effect of a magnetic filler, by a method wherein an electromagnetic coil is divided into plural parts and the part of the electromagnetic coil divided in a definite attenuated state is short-circuited to make the attenuation alternating current of the electromagnetic coil extremely small.

CONSTITUTION: The electromagnetic coil of an electromagnetic filter is divided into two electromagnetic coils 14A, 14B. In carrying out the demagnetization of a magnetic filler, the current control of a thyristor converter 28 is carried out under the action of a function generator 30 to supply an attenuation alternating current to the electromagnetic coils 14A, 14B. When a difinite attenuation state is estabilished, a short-circuit switch 32 is closed to flow the attenuation alternating current only to the electromagnetic oil 14A. As the result, because a magnetic flux amount passing through the magnetic filler can be reduced by half, demagnetizing effect can be enhanced.