A serial image forming apparatus comprising: a plurality of carriages (15D; 15U) each of which carries a liquid discharge head (16D; 16U); an acquisition unit configured to acquire divided print data divided from print data, the divided print data having an extended size extended in a direction corresponding to a conveyance direction of a recording medium from a size corresponding to a length of a nozzle array of the liquid discharge head (16D; 16U) in the conveyance direction, characterised in that the image forming apparatus further comprises: an identifying unit configured to identify, for the liquid discharge head (16D; 16U) of a particular carriage of the plurality of carriages (15D; 15U), a shifted data portion from the divided print data acquired by the acquisition unit , the shifted data portion having a size corresponding to the length of the nozzle array and being shifted by a number of pixels corresponding to an amount of deviation from a target installation position of the particular carriage in the conveyance direction; a discharge control unit configured to control the liquid discharge head (16D; 16U) to discharge ink to the recording medium using the shifted data portion identified by the identifying unit ; and a movement control unit configured to control the particular carriage to move in a main scanning direction based on the shifted data portion identified by the identifying unit , the main scanning direction being perpendicular to the conveyance direction. a plurality of carriages (15D; 15U) each of which carries a liquid discharge head (16D; 16U); an acquisition unit configured to acquire divided print data divided from print data, the divided print data having an extended size extended in a direction corresponding to a conveyance direction of a recording medium from a size corresponding to a length of a nozzle array of the liquid discharge head (16D; 16U) in the conveyance direction, characterised in that the image forming apparatus further comprises: an identifying unit configured to identify, for the liquid discharge head (16D; 16U) of a particular carriage of the plurality of carriages (15D; 15U), a shifted data portion from the divided print data acquired by the acquisition unit , the shifted data portion having a size corresponding to the length of the nozzle array and being shifted by a number of pixels corresponding to an amount of deviation from a target installation position of the particular carriage in the conveyance direction; a discharge control unit configured to control the liquid discharge head (16D; 16U) to discharge ink to the recording medium using the shifted data portion identified by the identifying unit ; and a movement control unit configured to control the particular carriage to move in a main scanning direction based on the shifted data portion identified by the identifying unit , the main scanning direction being perpendicular to the conveyance direction. an identifying unit configured to identify, for the liquid discharge head (16D; 16U) of a particular carriage of the plurality of carriages (15D; 15U), a shifted data portion from the divided print data acquired by the acquisition unit , the shifted data portion having a size corresponding to the length of the nozzle array and being shifted by a number of pixels corresponding to an amount of deviation from a target installation position of the particular carriage in the conveyance direction; a discharge control unit configured to control the liquid discharge head (16D; 16U) to discharge ink to the recording medium using the shifted data portion identified by the identifying unit ; and a movement control unit configured to control the particular carriage to move in a main scanning direction based on the shifted data portion identified by the identifying unit , the main scanning direction being perpendicular to the conveyance direction. <EOS>