An image data signal value conversion is performed such that the volume of ink applied to image areas on a print medium to be printed by the overlapping portions of the printing element substrates is smaller than that applied to image areas on the print medium to be printed by portions other than the overlapping portions. Alternatively, the image data signal value conversion is performed such that the volume of ink applied to overlapping areas on the print medium to be printed by two consecutive printing main scans is smaller than that applied to areas other than the overlapping areas. For this reason, even when forming photographic images by using an elongate “joining head”, high-quality images with no density unevenness between overlapping areas and other areas on a print medium can be produced. Also in a serial type printing apparatus, uniform images with no seam stripes can be produced.