Disclosed is an optical data recording/reproducing device which is provided with a laser diode for emitting laser beam, an illuminating optical system including an objective lens which converges the laser beam emitted by the laser diode on an optical disc, a monitoring optical system including a light receiving element which receives a part of the laser beam emitted by the laser diode, and a driving circuit which drives the laser diode in accordance with an output signal of the light receiving element. A coupling efficiency of the illuminating optical system and a coupling efficiency of the monitoring optical system are made substantially the same.