PURPOSE: To detect the large degree of optical modulation due to presence/ absence of pits on a medium by providing a light extracting member near the condensing point of a reflected beam from the storage medium to detect only the light near the optical axis of the optical beam.

CONSTITUTION: An aperture 35 as the light extracting member is provided near the condensing point of a projection lens 34 which condenses an optical beam L21 reflected from an information storage medium 10, and only the light near the optical axis of the optical beam out of the light passing the aperture 35 is detected by a photodetector 36. In this case, the diameter of a hole 35a is selected properly to receive a ≤1/1 quantity of the overall quantity of light. Thus, the bad influence of the reflected light, which is caused because the diameter of the minimum spot of the optical beam is larger than the pit diameter, is eliminated to detect the large degree of optical modulation due to presence/ absence of pits.