PURPOSE: To detect a speed of an optical head without adverse influence to a vibrating characteristic by using a driving coil of a linear motor.

CONSTITUTION: In a linear motor 31 having a driving coil 13 movable relatively to a magnetic member 31a in a direction crossing a magnetic flux generated by the member 31a, a relative speed of the coil 13 and the member 31a, i.e., a moving speed of an optical head 3 by the motor 31, is detected by utilizing an electrical change in the coil 13 generated at a moment of crossing the magnetic flux by the coil 13.