Abstract: A gradation data processing method for shortening printing time by processing a signal per bit column makes a heating element generate heat in the mid portion of one line printing time for a low gradation and makes the heating element repeatedly generate heat in all ranges during high gradation printing. The gradation data processing method includes steps for storing per line an N-bit picture data capable of expressing the 2.sup.N gradation, reading per predetermined bit column the N-bit picture data capable of expressing the 2.sup.N gradation and printing per the predetermined bit column the picture data read during the reading step while allowing the picture data to have respectively different heating time per the predetermined column.