PURPOSE: To reduce a bias in a regenerated waveform and improve the fidelity of an output waveform by providing a spiral groove along the surfaces of opposed polarities and filling an electroconductive element in the groove, thus constituting a closed circuit.

CONSTITUTION: Spiral grooves 10, 10' are provided on the surface along the respective clearances 3 of a central polarity 1 and an external peripheral polarity 2. Under this arrangement, it is possible to increase a magnetic resistance against a cross magnetic field along a magnetic pole surface. As a result, a disturbance of the magnetic field in a clearance 3 is reduced and the followup property of the drive force of a moving coil against the waveform of an electric current running through the coil is improved In this manner, a bias is reduced.