1. An image forming range varying system mounted on the image forming apparatus which jets an ink liquid to a conveyed image forming medium to form an image the system, comprising: an ink head unit comprising at least two head rows in which at least a plurality of ink heads are alternately arranged with a predetermined overlap in a substantially vertical direction with respect to a conveying direction along the conveying direction of the image forming medium; a first detection unit which is disposed on a supply, side of the image forming medium and which detects a tip portion of the image forming medium; a conveying mechanism which conveys the image forming medium and which sends conveying information of the image forming medium; a second detection unit which detects both transversal side ends of the image forming medium; an image forming range production unit which produces/outputs information of an image forming range with respect to the image forming medium based on information of the second detection unit; and a control unit which varies/controls an image forming operation based on pre-recorded appropriate image forming timing information and the information of the image forming range production unit.