PURPOSE: To detect accurately defects of printed matters by processing electrically the sensitivity characteristic to variable density of a photodetector and correcting the sensitivity characteristic to variable density before comparison and discrimination.

CONSTITUTION: The pattern of a printed face A is read by a sensor 10, and the output is inputted to an A/D converter E through an amplifier D and is converted to a digital signal. This digital signal is sent to a variable density sensitivity corrector P to correct the sensitivity characteristic to variable density of a photodetector 18, and thereafter, the signal is compared with a standard digital signal from a memory F by an operating circuit H. The output of the operating circuit H is compared with an allowance value from an allowance setter I by a comparing circuit J to discriminate error points. The output of the circuit J is given to a counter K, and a fraction defective is calculated by an operating circuit L and a fraction defective setter U.