In an ink-jet printing apparatus in which a carriage mounting an ink-jet printhead having a plurality of nozzles arrayed in a row of even-numbered nozzles and a row of odd-numbered nozzles for performing printing by discharging ink from the print elements is scanned back and forth over a print medium in a direction cross to the direction in which the print elements are arrayed to thereby print on the print medium, print data in conformity with the structure of the printhead is generated from entered information. Printing is carried out upon thinning print data, which is printed by the nozzles of the row of odd-numbered nozzles, if printing is performed by scanning the carriage in the forward direction, and upon thinning print data, which is printed by the nozzles of the row of even-numbered nozzles, if printing is performed by scanning the carriage in the reverse direction. This improves print quality by reducing edge roughness.