PURPOSE: To hardly deviate focus by providing a light shielding plate provided with a photodetector detecting a reflected light from an optical information recording medium on its rear side and having a prescribed shielding characteristic in the light path of an optical system.

CONSTITUTION: A laser spot light outgoing from a semiconductor laser 1 becomes the linearly polarized light of S polarization through a collimate lens 2 and is made incident on a beam splitter 3 and then the optical information recording medium 7 provided with the photodetector 8 on the rear surface is irradiated with the laser spot light through the light shielding plate 5, etc., having the light shielding characteristic more reducing the diffraction limited diameter of the light spot. Then a reflection back light spot by Kerr effect from the medium 7 is made incident on the photodetector 10 through a back light path. On the shielding plate 5 of a round shape, light shielding films whose transmission light quantity becomes the same quantity when the objective lens 6 focuses are provided on whose central part and whose outer peripheral part and the photodetector is arranged on a part where luminous flux diameter is large relatively and light quantity balance is hardly broken even when a relative position between liminous flux and the photodetector is slightly deviated and focus is hardly deviated.