PURPOSE: To eliminate the imbalance of attractive force on both sides of a disc, by detecting the flux density between the disc and the polar iron cores on both sides facing the disc surface and by controlling the output value to be equal.

CONSTITUTION: The gap flux density between the polar iron cores 11A and 11B on both sides of a disc 8 and the surface facing the disc is detected by flux density detecting elements 14A and 14B such as Hall elements, search coils, etc. The voltage of the separate DC current flowing respectively to the exciting coils 12A and 12B on both sides of the disc is controlled so that the flux density value on both sides of the disc 8 outputted from these detecting elements 14A and 14B may always be equal.