PURPOSE: To exactly appreciate a disk substrate in a short time, by inputting a scanning output of a video camera provided on a microscope for inspecting a disk to be measured, to a comparator, and detecting a defect by an output exceeding a prescribed level and the number of scanning lines.

CONSTITUTION: The surface of a disk 11 to be measured is displayed on a monitor TV14 by a video camera 13 provided with a microscope 12. An output of the monitor TV14 is inputted to a comparator 16 through an amplifier 15, is compared with a slice level, and when it is below its level, an output appears. On the other hand, the output of the monitor TV is inputted to a horizontal synchronizing clock generating circuit 17, its clock pulse C is applied to the comparator 16 through a gate circuit 18, and a signal from the comparator 16 is sent to an MPU25. Also, a signal synchronizing with the clock pulse C is outputted from a generator 20 and a vertical synchronizing clock generating circuit 19, is sent to the MPU25 through adders 21, 22 and registers 23, 24, and the disk substrate 11 is appreciated exactly in a short time.