A method comprises the step of reading multi level data of reference of each of colors from a printed paper on which images are printed to be good. The multi level data of reference are converted into two level data of reference so that two level data of images of reference can be reproduced in a memory from the two level data of reference. The method further comprises the step of reading multi level data of inspection of each of colors from a printed paper which is fed when inspecting. The multi level data of inspection are converted into two level data of inspection so that two level images of inspection can be reproduced in the memory from the two level data of inspection. The method further comprises the step of comparing the two level images of inspection with the two level images of reference for inspection of stained parts and blurred parts.