A printing apparatus comprising a printhead including two nozzle arrays neighboring in a first direction, each including nozzles arrayed along a second direction, a conveying unit for conveying a sheet in the first direction, a driving unit for driving the printhead so that each nozzle performs preliminary discharge on the sheet, and a unit configured to perform selecting some nozzles as non-driving nozzles and the remaining nozzles as driving nozzles for every column data in image data, determining image data to be assigned to each array as a first data so that a dot corresponding the non-driving nozzle in one of the two arrays is printed by the driving nozzle in the other, determining data for preliminary discharge as a second data so that each driving nozzle performs preliminary discharge, and ORing the first and second data.