PURPOSE: To make a trouble localization possible by utilizing output signals of a picture signal generating photo detector to perform the check for light source deterioration and further diagnosing whether the operation of a picture signal amplifier circuit is good or not on a basis of the output of a false picture signal generation circuit in the picture signal quantizing circuit of a picture signal generation unit.

CONSTITUTION: The light outputted from picture 2 dependently upon the light from light source 1 for picture is read by line measure sensor 3, and read information is converted to electric signals and is applied to picture signal quantizing circuit 20. The output is applied to picture signal amplifier circuit 5 through switching gate 4-1, and picture signals which are converted to a binary and are amplified by circuit 5 are outputted to output terminal 6. Then, these picture signals are applied to light quantity decision circuit 7 to decide the light quantity, and the decision result is outputted to light quantity decision terminal 8. Further, the output of false picture signal generation circuit 9 is applied to amplifier circuit through switching gate 4-2 at a self-diagnosis time of amplifier circuit 5, and the output of circuit 5 is applied to diagnosis decision circuit 10 to decide whether the operation is good or not, and the result is outputted to an amplifier diagnosis decision signal terminal.