In an image forming apparatus that uses a stackless ADU and executes image formation on both sides of a paper sheet in a facedown mode, a CPU confirms, when double-side image formation is to be executed with two paper sheets, image formation on reverse surfaces of which is finished, being stayed in the ADU, whether monochromatic image formation or color image formation is to be executed on an obverse surface of a first paper sheet that is stayed in the ADU and has a reverse surface on which image formation is finished. When the color image formation is to be executed, the CPU controls the image formation on the obverse surface of the first paper sheet that is stayed in the ADU and has the reverse surface on which image formation is finished. When the monochromatic image formation is to be executed, the CPU controls the image formation on a reverse surface of a second paper sheet that is stayed in the ADU.