When an image is formed on a second side of a sheet, the image forming position is adjusted on the basis of a value calculated using the amount αn of lateral registration deviation detected before forming an image on the first side and the amount γn-1, of lateral registration deviation of the previous sheet on which image formation is performed. Thus, it is possible to adjust the positions of irradiation of laser light to the photosensitive drums and to start forming the second-side images as soon as the first-side images have been primary-transferred onto the intermediate transfer belt.