PURPOSE: To reduce the difference in the detection accuracy due to variance in the density of a part to be inspected, by varying the detection sensitivity depending on the density of an image in a scan area of a printed matter to elevate the detection accuracy of a pattern portion.

CONSTITUTION: A frequency convertor section 9 converts the frequency of a fundamental pulse according to outputs of self-scanning type photoelectric conversion detectors 2a and 2b which each scans the same portion of the same graphic and applies the resulting pulse to the detectors 2a and 2b as scanning clock pulse. A comparator section 10 compares the output of a differential amplifier 5 for outputting differential signals of the detectors 2a and 2b with a reference area value and outputs a signal for decision on the propriety. In this manner, when the density of an image increases in a scan area of a printed matter with the convertor section 9 and the comparator section 10, the output level of the self-scanning type photoelectric convertor is augmented with respect to the same input level. This can elevate the detection accuracy for a pattern portion with limited reflectance thereby reducing the difference in the detection accuracy due to variance in the density of a part to be inspected.