PURPOSE: To make an optical head small-sized and lightweight and to reduce the heat generation by constituting a magnetic circuit consisting of magnetic poles and a permanent magnet in the position facing the optical head through a medium and moving the whole of this magnetic circuit to switch the direction of the magnetic field.

CONSTITUTION: An object lens 1 for focusing is provided, and an optical head 3 has a focusing mechanism and irradiates laser light so that it is always focused onto a medium 4. In case of recording of information on the medium 4, the optical head 3 is moved to a prescribed position, and the whole of the magnetic circuit (consisting of a magnetic poles 6 and 7 and a permanent magnet 8) is moved to a state (a) by a driving device 6, and the medium 4 is magnetized downward by irradiation of the laser light. In case of erasing of information on the medium 4, the optical head 3 is moved to a state (b), and the medium is magnetized upward by irradiation of the laser light. Since a device for applying an external magnetic field is constituted of the permanent magnet 8 and magnetic poles 6 and 7 and is placed on the side opposite to the medium 4 of the optical head 3, a conventional solenoid coil 2 is not provided in the optical head, and the head is made small-sized and lightweight.