This invention has as its object to realize multi-pass print control which requires a short processing time by a simple arrangement. To this end, a printer according to this invention attains multi-grayscale print control by changing the number of dots to be printed on each pixel to overlap each other in multi-pass print control that prints the pixel in a plurality of scans. In this case, index patterns, each of which designates which of scans is to be used to print in accordance with the density level of a pixel, are prepared for respective pixel values. If a plurality of patterns are prepared for each pixel value, a pattern is randomly selected from them. An image is printed in accordance with a print pattern defined by the selected pattern.