Abstract: A system for a magneto-optical information recording/reproducing system has a polarizing beam splitter for splitting a light beam reflected by a magneto-optical disk, which utilizes the Kerr effect, into one light beam for detecting focussing errors and two light beams for detecting tracking errors. The polarizing beam splitter operates to split a polarizing light beam in such a manner that the two light beams for detecting tracking errors have a polarizing beam composition that is perpendicular to the polarizing beam composition of the light beam for detecting focussing errors. The optical system includes a light receiving element for receiving the light beam split by the beam splitter. The light receiving element has a light receiving portion for detecting focussing errors and a light receiving element for detecting tracking errors. The light receiving portion for detecting focussing errors has a plurality of light receiving areas for receiving the light beam for detecting focussing errors.