PURPOSE: To detect assuredly the out-of-focus, by providing a light transmission limiting membe at the front or rear stage of an extracting member that extracts the reflected light given from an image forming layer in order to cut off the higher order terms of a Fourier pattern opposite to a pattern on the image forming layer.

CONSTITUTION: The laser light 11 is condensed on an information forming layer 4 via a coupling lens 13, a polarized beam splitter 15, a λ/4 plate 16 and an objective lens 17. The reflected light travels backward to pass through the splitter 15, and the higher order terms of a Fourier pattern are cut off by a light transmission limiting member 27 for the light 11. Then the light 11 is formed into an image on a 2-split photodetector 26 through a light extracting member (knife edge) 23 set at the position out of the optical path and a lens 24. Then the out- of-focus is detected by detecting the shift of a beam spot by photodetecting cells 26a and 26b. The effect of diffraction due to the fine recessed and projected surface of the layer 4 is reduced since the higher order terms of the Fourier pattern are cut off. Thus the out-of-focus can be assuredly detected.