A magneto-optic overwrite disk recording system capable of canceling shifts of the front edge and the rear edge of the record mark due to accumulation of heat. The recording system includes a magneto-optic overwrite disk having a memory magnetic film and a record magnetic film, a first magnet for applying an initialization magnetic field with a first strength vertically to the disk, and a second magnet for applying a bias magnetic field with a second strength smaller than the first magnetic field to the disk in the direction opposite to the initialization magnetic field. The recording system further includes a laser diode for generating laser beams, an optical head for focusing the laser beam on the disk, a pulse generator for generating square wave pulses with a predetermined period, and a duty ratio controller for alternately increasing and decreasing the duty ratio of the square wave pulses every time data "1" of binary record data is generated. The laser diode is driven by a constant-current circuit for write power. There is connected a switch between the laser diode and the constant-current circuit for write power. The switch is turned on/off in accordance with the output pulse of the duty ratio controller so that the laser diode emits a pulsed beam.