Mask patterns to be used for multi-pass printing make it possible to make a print with a higher quality at a higher speed. Specifically, by performing a swapping process with adjacency forbiddance, two points in the horizontal direction, or the scanning direction of a printing head, are selected in a buffer in which codes for each scan pass are set depending on printing ratios of a gradation mask. Subsequently, codes are swapped between the two points. By this swap, adjacencies between print permitting areas are eliminated in the mask pattern. As a result, when driving frequencies set for the printing head is kept constant, the scanning speed can be doubled at minimum.