PURPOSE: To achieve a smaller size of a magnetic and a suppression of a power consumption by keeping a working distance between a disc and a bias magnet constant in a short range.

CONSTITUTION: A bias magnet unit has a bias electromagnet 1 and the bias electromagnet 1 is supported on a stay 4 integral with an optical head 5 through a support part 3 by a positioning means 2. The supporting position of the bias electromagnet 1 is such that an optical axis of an objective lens of the optical head almost coincides with a center axis of a magnetic pole sandwiching a photomagnetic disc 7. The means 2 makes the magnet 1 displace with a servo mechanism to keep a working distance constant between the electromagnet 1 and the disc 7. The servo mechanism inputs a focus error signal from a focus error detection circuit into an AGC10, a phase compensation circuit 11 and a piezo-electric element driving circuit 13 sequentially to deflect an actuator 2 thereby allowing the electromagnet 1 to follow a displacement of a disc surface.