PURPOSE: To simplify and miniaturize a device by attaching a dielectric multilayered film to the incidental face of a light beam reflected by the recording medium of a first prism constituting a photodetector, and forming a beam splitter with the multilayered film.

CONSTITUTION: Three light beams L1, L2 and L3 provided by an optical element 4 are made incident through a condenser lens 7, convex lens 8 and cylindrical lens 9 to a photodetection unit 10. Namely, the element 4 is functioned as the beam splitter for splitting a laser beam Li made incident from a light beam generating source 1 through an objective lens 5 to a magneto-optical disk 6 and a reflected laser beam Li returned from the disk 6 through the lens 5 with a dielectric multilayered film 43 formed on the incidental face part of a first prism 41. At the same time, the element is functioned as the photodetector for splitting and emitting the beam Li into the beam L1 of a P deflected component, the beam L2 of an S deflected component and beam L3 of the synthetic component of P and S deflected components with first and second prisms 41 and 42.