A sheet which is again conveyed to an image forming section passes through a re-feeding passage. A reference member is provided on one end of the re-feeding passage which is perpendicular to a sheet-conveying direction, and is provided with a plurality of reference surfaces in a stepwise manner. Side ends of sheets having different sizes and passing through the re-feeding passage abut against the reference surfaces to adjust positions of the sheets in widthwise direction. The re-feeding passage is provided with obliquely conveying means which obliquely conveys the sheets having different sizes while pushing the sheets against the reference surfaces. A support member which is provided upstream of the obliquely conveying means supports the sheet such that one end of the sheet supported by the support surface extending from lower ends of the reference surfaces substantially horizontally and the other end of the sheet have substantially the same heights.