PURPOSE: To enable the detection of defective parts from a printed matter moving fast by providing two image sensors adapted to take first and second patterns to be inspected each time a printing machine moves by a fixed value to perform a special processing of video signals therefrom.

CONSTITUTION: When defective parts are detected from same patterns P1 and P2 printed repeatedly in two rows along the length of a printing paper 1 thereon moving at a constant speed lengthwise, image sensors 2 and 3 are provided separately on the running ways of the patterns P1 and P2 to determine the difference based on video signals from both the image sensors 2 and 3 with a subtraction circuit 6. Then, it is judged with level discriminator circuits 7 and 9 whether the level of the difference is within a specified range or not.