PURPOSE: To inspect improper operation of amplifiers connected to read-in photodetectors for card readers, by giving light to all the photodetectors at the same time judging the response of the amplifiers.

CONSTITUTION: The photodetectors receive photoradiation at the same time, and the output of the amplifier at that time is input to the OR circuit 1 and the NAND circuit 2. The output (a) of the OR circuit 1 is at 1 with the output of fastest amplifier. The output (b) of the NAND circuit 2 is at 1, if no output is obtained from one amplifier and at 0 is output is obtained from all the amplifiers. After the output (a) is at 1, the detection pulse generating circuit 3 makes the output (d) to 1 after the time preset as the range of error of instantaneous operation (mainly rising time) of the amplifier. The AND gate 4 outputs 1 if the output (b) is as yet 1 when the output (d) is at 1 to set FF5 and to output error output. Thus, the defective operation of amplifier can be konwn.