A one-pass system image forming apparatus for forming an image on both sides of a recording medium is capable of easily suppressing image disturbance caused when an unfixed toner image suffers abrasion when a sheet of transfer paper is conveyed from double-sided transfer means to a fixing device, and image disturbance caused when toner that has become adhered to a guide portion for guiding the transfer paper from the double-sided transfer means to a fixing nip of the fixing device is transferred to a subsequent sheet of transfer paper.