An image forming apparatus includes a reverse side print mechanism for printing on the reverse side of continuous paper, a surface print mechanism for printing on the surface of the continuous paper, and a width detecting mechanism for detecting a width of the continuous paper. The apparatus further includes a controller which, based on the detected paper width, determines a print start position on the reverse side of the continuous paper and controls the reverse side print mechanism to start printing from the determined print start position. Thus, even if the paper width is changed, the reverse side print start position can be set to a predetermined position.