PURPOSE: To attain to simplify a system and to obtain a detection signal with high reliability, by detecting a material to be inspected by two reflection type photosensors provided to the underside of a conveyor line of the material to be inspected.

CONSTITUTION: Gaps 4 are provided mutually between rollers 2 of a conveyor line 1 for sending a material 3 to be inspected. A first and a second reflection type photosensors 6a, 6b are one for detecting the presence of the material to be inspected by reflected rays generated by irradiating infrared rays to said materials to be inspected and attached to a sliding mechanism 7 provided to the underside of the conveyor line while the sliding mechanism 7 is supported by supports 8a, 8b. When the slide mechanism 7 is locked at a predetermined height, the upper ends of the photosensors 6a, 6b are positioned slightly below the upper curved surfaces of the rollers 2 within the gaps 4 between the rollers and the positions thereof are determined in no contact relation to the surface of the material to be inspected. Therefore, when the material 3 to be inspected is sent, the photosensors 6a, 6b are successively turned ON and the obtained detection signals are imparted to a synchronous pulse generator 9 to be pulsated thereby while formed into synchronous pulse 10 to be imparted to an ultrasonic flaw detector 5.