PURPOSE: To prevent a magnetic field generating means from being moved and made to generate heat uselessly and thereby to reduce consumption of power and generation of heat by prohibiting movement of a magnetic field generating means without fail when the movement thereof is unnecessary.

CONSTITUTION: A first moving means 120 for movement in the direction of a track width of a light beam medium, a magnetic field generating means 130 for generating a magnetic field in a limited sphere, a second moving means 140 for moving the magnetic field generating means 130 in the direction of the track width of the medium discretely from the first moving means 120, and a switch means 100 for prohibiting the operation of the second moving means 140 on the basis of information on detection from the employed medium side, are provided. When the operation of the magnetic field generating means 130 is unnecessary, the operation is prohibited without fail. Thereby a state wherein the magnetic field generating means 130 is moved and made to heat uselessly is avoided without fail and consumption of power and generation of heat can be reduced.