PURPOSE: To identify the kind of a defect based on the relative relationship of three outputs by casting circularly polarised light on the surface of a disk, separating and detecting part of the reflected light, separating and detecting scattered light and diffracted light, and separating and detecting the positive reflected light from the surface of the disk.

CONSTITUTION: Laser light 1 is made to be circular polarized light through a 1/4 wavelength plate 2. The light is cast on a hard-disk surface 4 through a lens 3. The reflected light is separated with a half mirror 6 through a lens 5. The light introduced into a first detector 9 through a lens 7 and an analyzer 8. Part of the light transmitted through the mirror 6 is reflected from 10 having a strip-shaped mirror vapor-deposition part. The light enters into a third detector 11 for detecting the regularly reflected light and a fourth detector 12 for detecting the diffracted light from a defect. The transmitted light through the mirror 10 further enters into a second detector 14 through a lens 13. The outputs of the detectors 9, 14, 11 and 12 become the first-fourth defect outputs through a photoelectric conversion and output circuit and a comparator, respectively. Sputter missing, attachment of foreign matter and the like are identified based on the correlation of the first-third outputs. The linear flaw in the circumferential direction is detected based on the fourth output.