PURPOSE: To prevent printed matter from being judged to be inferior on the basis of a slight hue change sufficiently passing as a commodity.

CONSTITUTION: A hue change confirmation circuit 3 is provided. In this circuit, a grouping memory controller 31 takes in the reference image data stored in a reference memory 6 and divides image data of an intermediate level into a plurality of groups. A difference sum total circuit 32 inputs the difference data from a difference circuit 9 and accumulates and averages the absolute values of the difference data of the intermediate level of every group according to group division to calculate a group average. A comparator circuit 33 confirms that a hue is changed when the group average exceeds a prescribed value to generate a reference value memory signal to newly perform the renewal of the reference image data and group division.