PURPOSE: To make a recording domain, which is apt to contract, more difficult to change at the time of the rise of temperature due to light beam irradiation for reproducing by detecting the start of a series of recording operations or the end of the erasing process to quickly invert or re-invert a bias magnetic field.

CONSTITUTION: A recording operation start detecting means 2, a recording operation end detecting means 3, a bias magnetic field control means 4 are incorporated in a controller 1. In this constitution, the magnetic field is inverted at the time of the start of a series of recording operations, where erasing and recording are performed in order, and at the time of the end of the erasing process of this series of recording operations. That is, the magnetic field is inverted only just before and after erasing, and as the result, the magnetic field is always applied in the same direction at the time of recording and reproducing.