1. A method of performing duplex printing, comprising: printing 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 the next image sequentially printed on the first surface; reversing the sheet where the plurality of images are printed on the first surface; reading the reference mark recorded on the first surface of the reversed sheet; setting a position to be printed of an image on a second surface which is a back of the first surface, on the basis of the reading of the reference mark; and printing a plurality of images on the second surface.