An image processor unit minimizes image-quality degradation that can be caused by the addition of predetermined information to an image and that enables the predetermined information to be added to the image so that embedded information can be accurately extracted. The image processor unit includes an input means for inputting the image; a plurality of quantization-threshold setting means, of which occurrence probability distributions are different from one another, for setting quantization thresholds in terms of probability; a control means for controlling a regularity for selection of the plurality of quantization-threshold setting means in units of a predetermined image region according to the predetermined information; and a quantizing means that uses quantization thresholds set by the quantization-threshold setting means and thereby quantizes the image according to an error spreading method.