1. A method of performing duplex printing, the method comprising: printing, by an inkjet head, a plurality of images on a first surface of a sheet, wherein the sheet is continuous; recording a reference mark in an area between one image and a next image sequentially printed on the first surface; reversing, by a reverse unit, the sheet where the plurality of images are printed on the first surface and feeding the reversed sheet for printing by the inkjet head again; performing, by a decurl unit, decurling of the sheet fed from the reverse unit; reading the reference mark recorded on the first surface of the reversed sheet fed from the reverse unit through the decurl unit; setting, based on the reading of the reference mark, a position to be printed of an image on a second surface which is a back of the first surface; printing, by the inkjet head, a plurality of images on the second surface; recording a cut mark in a margin area between one image and a next image sequentially printed on the second surface based on the reading of the reference mark; detecting the cut mark on the second surface within a limited search range which is narrower than the margin area and is set inside the margin area; setting a first cut position and a second cut position for cutting off the margin area in response to the detection of the cut mark within the limited search range, or setting the first cut position and the second cut position by estimating a position of the cut mark based on information on already detected cut mark in response to no detection of the cut mark within the limited search range; cutting the sheet at the first cut position and the second cut position; and discharging each of cut sheets on which the images are formed both on the first and second surfaces.