An image forming apparatus includes image forming means (1) configured to form an image onto each sheet, ejection means (5) on which the sheet with the image formed by the image forming means is ejected, sorting means configured to sort the sheets to be ejected on the ejection means into subsets, and control means (7) configured to, when executing a mode for forming images on first sides of sheets, ejecting the sheets with the images on the first sides, and forming images on second sides of the ejected sheets manually placed on sheet feeding means for second-side image formation, control the sorting means to sort the sheets ejected on the ejection means after first-side image formation into subsets so that the number of sheets in each subset does not exceed a maximum number of sheets stackable on the sheet feeding means.