A CPU generates a mask pattern by generating patterns corresponding to a predetermined number of times in the sub-scanning direction in accordance with the number of printing elements of a printhead which corresponds to the amount by which scanning is performed once in the sub-scanning direction, and repeating this operation by the number of times corresponding to a predetermined number of columns. The generated mask pattern is stored in an RAM. An image is then printed by using image data and the mask pattern stored in the RAM for each printing timing signal in the main scanning direction.