PURPOSE: To improve the reliability of a device by deciding a defective accurately by storing a defective signal obtained by discriminating an inspection signal obtained by optically scanning the overall width of a material to be inspected, and by displaying and recording a still picture in the center of a monitoring device.

CONSTITUTION: The overall width of a material to be inspected is scanned optically and output signals of photodetectors 4 and 4a are inputted to a discrminating circuit 10 through a preamplifier 7, a main amplifier 8 and a signal processing circuit 9 to discriminate and store a defective signal. On receiving a synchronizing pulse outputted by a pulse generator 23 each time one line is scanned, a control circuit 24 writes the signal from the amplifier in a frame memory 25 through a switch circuit 21 and an ADC22. At the same time, a signal from the circuit 10 is sent to a control circuit to perform address movement processing. After the processing, the contents of the memory 25 are passed through a DAC28 to be monitored 26 as a still picture so that a defective picture is in the center, and defective data from the discriminating circuit 10 is recorded 31 through a gate circuit 30. Thus, the defective is decided accurately and the reliability of the device is improved.