PURPOSE: To attain to simplify a system and to carry out detection with high reliability, by detecting a material to be inspected by two transmitter and receiver type ultrasonic sensors having transmission and receiving capacity provided to the underside of a conveyor line to the material to be inspected.

CONSTITUTION: Gaps 4 are provided mutually between rollers of a conveyor line 1 for sending a material 3 to be inspected. A first and a second transmitter and receiver type ultrasonic sensors 6a, 6b are one for projecting ultrasonic waves to the material to be inspected on oneself and receiving reflected waves therefrom and attached to a sliding mechanism 7 provided to the underside of the conveyor line 1 while the sliding mechanism 7 is supported by supports 8a, 8b. When the sliding mechanism 7 is locked at a predetermined height, the upper ends of the sensors 6a, 6b are positioned slightly below the upper curved surfaces of the rollers 2 within the gaps 4 between the rollers 2 and the positions thereof are determined in no contact relation to the surface of the material to be inspected. When the material 3 to be inspected is sent, the sensors 6a, 6b are successively turned ON. The obtained detection signals are imparted to a synchronous pulse generator 9 to be pulsated thereby and formed into synchronous pulse 10 to be imparted to an ultrasonic flaw detector 5.