Patent ID: 7396106

Claim:
An image forming apparatus in which an ink jet printing device is provided, the ink jet printing device comprising: a head unit in which a plurality of recording heads having discharge nozzles which discharge ink drops of different colors respectively are arranged in a main scanning direction, and the discharge nozzles of each of the recording heads are arranged at equal intervals in a sub-scanning direction which is perpendicular to the main scanning direction; a head-unit moving unit moving the head unit in the main scanning direction along a printing region of a recording medium; a recording-medium moving unit moving the recording medium in the sub-scanning direction; and a control unit controlling the head unit, the head-unit moving unit, and the recording-medium moving unit, wherein the control unit is configured to carry out printing procedures including steps of moving the recording medium to the printing region to perform the discharging of the ink drops, and moving the recording medium in the sub-scanning direction by an interval represented by H/k where H is an array interval of the discharge nozzles in the sub-scanning direction and k is an integer greater than one, to perform the discharging of the ink drops to a non-printed region equivalent to the array interval H of the discharge nozzles, and configured to repeat the printing procedures k times according to an ink dot density, and thereafter move the recording medium in the sub-scanning direction so that the head unit is located to a next non-printed region following the printing region and the printing procedures are performed for the next non-printed region, wherein the control unit is configured so that a rear-end portion of the discharge nozzles of the k th printing procedure in an array direction after printing of the printing region and a front-end portion of the discharge nozzles of the k th printing procedure in the array direction before printing of the non-printed region overlap each other with respect to the sub-scanning direction, and invalid nozzles that do not discharge the ink drops are determined from among arbitrary ones of the discharge nozzles including in the overlap between said front-end portion and said rear-end portion in the array direction of the discharge nozzles.