PURPOSE: To shorten the inverting time of a bias magnetic field by supporting a bar-shaped magnet magnetized into double poles in a width direction so as to be turnable around its longitudinal axis, and rotating the bar-shaped magnet until it strikes a stopper member by passing a current through a coil.

CONSTITUTION: When coils 2, 3 are energized so that the lines of magnetic force of the direction E are generated, two S-poles of the magnet 1 are attracted by the coils 2, 3 respectively, and N-poles are repulsed by the coils 2, 3, and the magnet 1 is rotated in the direction G1 around the axis 6. As a result, the magnet 1 is brought into contact with plates 4, 5, and is stopped as being kept in an inlined state. In such a state, the line of magnetic force due to the magnet 1 and the line of the magnetic force due to the coils 2, 3 are summed, and as a result, a bias magnetic field in the direction Z1 is given to a magneto- optical disk 18. When the current flowing through the coils 2, 3 is reversed, the magnet 1 stopped as being inclined in a reverse direction. In this state, the bias magnetic field given to the magneto-optical disk 18 becomes in a reverse direction Z2. Magnetic force generated by the coil can be used as driving force to rotate the magnet 1, and the inverting time of the magnetic field can be shortened.