PURPOSE: To detect surely defects other than pits on the surface of a disk by detecting rugged flaws other than pits on the surface of the optical disk with a photodetector and a comparator.

CONSTITUTION: If a defect exists on the surface of a disk 1', a photodetector 6a which has a maximum value of the quantity of reflected light due to this defect receives almost all of the quantity of reflected light due to the defect. The quantity of a diffracted light A' due to diffraction of a pit on the disk 1' is little because the light receiving area of the photodetector 6a is small. Thus, a ratio of the quantity of diffracted light due to the defect to the photodetector 6a to the quantity of diffracted light due to the pit to the photodetector 6a is made higher to improve the S/N. Only the output of the photodetector 6a is outputted to a logical adder 9 with a high S/N if the output of the photodetector 6a corresponding to the maximum value of the reflected light due to the defect is higher than a set voltage of a comparator 8, and thus, defects other than pits on the surface of the disk are detected surely.