PURPOSE: To prevent the incidence of return light to a laser beam sorce by arranging a Faraday effect element, having a plane of polarizatin rotatable by 45 degrees, between a couple of polarizing prisms whose transmission axes are set at 45 degrees.

CONSTITUTION: On the optical path of laser beam polarizing prisms 5 and 7 whose transmission axes are set at 45 degrees are provided and between the two, Faraday glass 6 whose plane of polarization is rotatable by 45 degrees is arranged. In this constitution, the laser beam is passed through the prism 5 to obtain linear polarized light, which is turned by 45 degrees by the Faraday glass 6 to strike a recording track 23 by way of the prism 7, so that its reflected light is guided to a photodetector 15 through a beam splitter 8. In this case, the elliptic polarization component of the reflected light are removed by the prism 7 and its azimuth angle is increased by the Faraday glass 6 by 45 degrees to have a 90 degrees azimuth angle to the transmission axis by the prism 5. Therefore, it never returns to a laser beam source 3 and is reflected in a direction C, thus preventing the incidence to the light source.