PURPOSE: To detect precisely the bad printing like spot, by sending error signal, when the differential value of the picture information exceeds the preset value, if that differential information signal does not coincide comparing with the corresponding differential information signal in the memory.

CONSTITUTION: The detected signal of printed matter out of the light receiving element 12011 is differentiated by the differential circuit 401 and that signal is compared with three kinds of preset values of presetter 405W407 by the comparator 402W404, and the signal which exceeds the preset value makes output to gate 411W 413 via single stable multi 408W410, along with inputs to address controller 414. By that signal AR2, the count signal CV of decoder 200 inputs to comparing circuit 422W424 via contact point (b) of the change-over circuit, along with the position signal MP1WMP3 from the memory 421 inputs also. If both coincide, the signal from the single stable multi 425W427 inputs to the gate circuit 411W413, and when both input signals do not coincide, error signal ER3WER5 are developed. Thus it is able to detect precisely the bad printing like spot.