PURPOSE: To detect a fault with no interference with a thin film, by rotating a disc covered with a thin film so as to radially scan the disc with a laser beam, and by electrically converting film interfering light from the disc so as to obtain an electrical signal for the scanning in order carry out comparison.

CONSTITUTION: A disc 5 covered with a thin film is radially scanned with laser beam from a laser source 1 by means of a scanning optical system 2 as scanning means composed of a polygon mirror 3 and a projecting lens 4. Light reflected from the disc 5 is led through a light receiving lens 8 onto a photoelectric converter 10 so as to be converted into an electric signal which is processed by signal processing means 11 so as to detect a fault on the disc 5. A turn table 6 rotates the disc 5. Since the disc 5 is radially scanned with the laser beam, the period of an electrical signal delivered from the converter 10 is constant, and accordingly, it is possible to facilitate the signal processing such as comparison and computation by the processing means.