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.