PURPOSE: To inspect the information recording section of each sector for fault by detecting a boundary area and masking the detecting signals for the information bit of the boundary area and index data of each sector using mask signals synchronous to the detected boundary area.

CONSTITUTION: A laser beam is projected on an optical disk 1 from a laser light source 3 and the level change of the reflected beam of the laser beam is received by a photoreceptor 6. Then a boundary detector 7 detects an optically nonreflecting boundary area from changes in the light receiving level and outputs boundary signals and a monostable multivibrator 8 generates mask signals by using the boundary signals as a trigger. The scattered rays of light on the surface, on the other hand, are received by a photoreceptor 5 through a condenser lens 4 and the fault signal of an aimed information recording section, scattered rays of light P1 and P2 at both ends of the boundary area, and information bit signal IDX of index (IDX) data produced by the scattered rays of light are outputted. The output signals are inputted to a mask circuit 10 through a delay circuit 9 and the signals P1 and P2 and IDX are masked. Therefore, only the fault signal is inputted to a fault detecting section 11 and the fault of the information recording section can be inspected efficiently.