PURPOSE: To overwrite with easy control operation by arranging a magnet- optical disk in an electromagnetic field where the standing waves of electromagnetic waves exist and irradiating the magnetic layer of the disk with a laser spot when the magnetic field in the desired direction acts.

CONSTITUTION: A magneto-optical disk 6 is rotated and μ waves are radiated from an oscillator 1. These waves are made incident through a μ waveguide 2 on a cartridge 5 and move back and forth between this cartridge and the waveguide 2. The oscillation frequency and the distance between the oscillator 1 and the cartridge 5 are so selected as to generate the standing waves at which the amplitude of the magnetic fields is maximized within the seek range of an optical pickup 7. A laser diode of this pickup 7 emits a laser beam by driving pulses. The magnetic layer of the disk 6 is heated and coercive force is lost. The magnetic layer is magnetized by the magnetic field impressed thereto, by which information pits of an N pole or S pole is formed. The information pits are irradiated with the laser beam which is linearly polarized light. The reflected light is rotated in the plane of polarization according to the manetization direction of the information pits. The recorded information is reproduced by detecting the rotating direction.