PURPOSE: To enable to inspect the failed shape without being effected with noise, by judging the presence of failure with the detection signals of the differential detection of the reflection light from inspected object with a pair of detection elements and with one detection element.

CONSTITUTION: The reflection light from mobile inspected object 1 is detected with a pair of photo electric conversion elements 4, 4' having orthogonal slit to the direction of movement and with one photo electric conversion element 5 respond ing to unidirectional to the positive and negative direction. The detection output of the elements 4, 4' is processed with the differential amplifier 8 the level of which is greater at high speed and acceleration and deceleration of inspected object 1 in addition to failure, and the detection output of the element 5 is high for the level in reponse to the failed surface, not responding to the speed and acceleration and deceleration. When the comparators 12, 15 producing output with the input more than a given level, and the logic element 16 can judge the failure without being effected with noise when the differential detection output of the elements 4, 4' and the high level detection output of the element 5 are both present. Accordingly, the shape of failure can also be judged by using the differential detection photo electric conversion element pairs of a plurality different in pitch.