PURPOSE: To perform an always stable signal detection with high reliability by providing polarization separating elements on opposite surfaces held between the surfaces of two gratings respectively and arranging photodetectors on individual optical paths of transmitted light and diffracted light.

CONSTITUTION: Reflected light (a) from an optical information recording medium 7 is incident upon the 1st grating 28 of an optical path separating element 4, so that the transmitted light T and the diffracted light K are separated from each other, and these two kinds of light T and K are reflected by a surface formed as a total reflection film 3a respectively and then passed through the 2nd grating 29 to be emitted outside, and are detected by the photodetectors arranged on the optical paths respectively. Consequently, the lengths of the optical paths for the transmitted light T and the diffracted light K from their separation by the 1st grating to their detections by the photodetectors 31 and 32 are both equalized, and their spot diameters on the photodetectors 31 and 32 on the same surface are also equalized with each other. By this method, the always stable signal detection with high reliability can be performed without necessity of making the fitting accuracy for the photodetectors severe.