In order to form a high-quality image, upon execution of multi-pass printing using a printing head with a plurality of nozzles, an image processing apparatus calculates, using a scan duty setting unit, scan duty data for respective nozzles for each scan of the printing head in accordance with input image data. A halftone processor generates a dot pattern to be formed by applying N-ary processing based on predetermined constraining condition information to the calculated scan duty data. A constraining, condition information calculation unit generates constraining condition information to be referred to by the halftone processor at the time of the next scan. The constraining condition information is set so that the phase of a next dot pattern to be generated has an opposite phase relationship in a low frequency region with respect, to the already printed dot pattern.