PURPOSE: To obtain an inexpensive reproducing device of simple structure which causes no loss of the quantity of light by using a polarization beam splitter and a quarter-wavelength element, converting linear polarized light into circular polarized light and irradiating a photomagnetic material of vertical magnetism with the light, and reconverting the reflected light into linear polarized light and detecting the quantity of the reflected light.

CONSTITUTION: The light polarized light incident from a light source to the polarization beam splitter 14 is transmitted efficiently, and reflected by the polarization beam splitter 14 by about 100% because the reflected light from the photomagnetic material 18 is made perpendicular to the linear polarized light from the light source by the quarter-wavelength plate element 16. Therefore, the loss of the quantity of light caused when the linear polarized light from the light source is transmitted through the polarization beam splitter 14 and when the reflected light is reflected is improved. Further, the need for an analyzer, differential detector, etc., for detecting the rotation of the oscillation plane of the linear polarized light of the reflected or transmitted light from the photomagnetic material is eliminated, and the device is constituted easily at a low cost.