An adverse effect of air flows occurring near the nozzle line with high print duty is reliably avoided by appropriately dividing and allocating the print data to a plurality of ink ejection nozzle lines. When the print head is scanning in a forward direction, a combination of a magenta ink nozzle line and a yellow ink nozzle line that do not adjoin each other is mainly used. When the print head is scanning in a backward direction, another combination of a magenta ink nozzle line and a yellow ink nozzle line that do not adjoin each other is mainly used.