PURPOSE: To erase residual magnetism which remains in a magnetic material core, and to eliminate distortion in case of reproduction, by applying an AC signal of prescribed frequency for a prescribed time, to a power genrating coil of a cartridge, and after that, gradually attenuating its signal to a zero level.

CONSTITUTION: To a power generating coil 8 of a cartridge having a magnetic material core, a constant-current output amplifier is connected, and the impedance to the coil 8 is matched. On the input side of this amplifier, an attenuating means 4 using an FET, and an AC signal applying means 3 are provided, bias voltage of the FET of this attenuating means 4 is controlled by charging voltage of a capacitor 5 connected to a switch 6, and the FET is shifted to a cut-off state from a conducting state in accordance with an operation of the switch 6. Subsequently, an AC signal of prescribed frequency from the means 3 is applied to the coil 8 for a prescribed time, and after that, the AC signal is gradually attenuated to a zero level by the attenuating means 4, residual magnetism of the magnetic material core is erased, and distortion of a reproduced sound due to the residual magnetism is eliminated.