PURPOSE: To make small an instantaneous relative speed between a magneto- optic disk and a light beam and improve reliability of recording and reproducing data even in case the data transfer speed is high.

CONSTITUTION: A diffraction element 108 arranged on a light beam path provided between a semiconductor laser 106 and a magneto-optic disk 110 to have a groove to diffract the laser beam, a motor 107 for rotating the diffraction element 108 and a motor drive circuit 105 are comprised. In the diffraction element 108, the groove located on the light beam path is formed orthogonally crossing the tracks on the magneto-optic disk 110 and rotation of the diffracting element 108 deflects the diffracted light beam along the track 110a to make small the instantaneous relative speed of the magneto-optic disk 110 and light beam.