4 has its one end connected to each one of terminals of negative power supply 5 and positive power supply 6 and the other end connected to a movable contact point 7a of 3 selection change over switch 7 that constitutes control means for controlling the strength of the external magnetic field the change over switch 7 as above described comprises three fixed contact points 7b 7c and 7d in which the fixed contact points 7b and 7d are connected to the other terminals of positive power supply 6 and negative power supply 5 respectively and its change over switching operation is controlled by means of a switch control circuit 11 the switch control circuit 11 controls the operation of the change over switch 7 depending on record data in such a configuration when the magneto optical disk 3 rotates at a constant rate opposed to the magneto optical recording head which holds the objective lens 2 in the optical system and the electromagnetic coil 4 in the magnetic modulation control system the change over switch 7 as above described performs the change over operation corresponding to a control signal given from the switch control circuit 11 to connect the movable contact point 7a to the fixed contact point 7b for a positive finite current value e g e to the fixed contact point 7d for a negative finite current value e g e or to the fixed contact point 7c for a zero current value though the change over switch 7 as above described is in practice not a mechanical switch but non contact switch means semiconductor switch.