PURPOSE: To prevent disorder of stacked transfer sheets caused by times of inversion, and to facilitate their arrangement, by selecting the order of transfer in accordance with times of inversion in the cource of conveyance of the sheets on both sides of which images have been formed.

CONSTITUTION: An image formed on an insulating drum 14 is transferred onto a sheet 18 and fixed with a fixer 23. Then it is decided whether to move the sheet forward from a first route changing means 24 to an inversion mechanism 28 to excute both side copying, or to lead the sheet with a second route changing means 25 to a discharge tray 26 on the outside of a copying machine, or to lead it to a sorter 27. For example, when both side copying is completed, the second side of the sheet 18 is directed upward, and left not inverted till the last tray, the following order of transfer operations. An image corresponding to the first original A is formed on the first side of the sheet 18, and the image of the second original B is formed on the second side, thus permitting disorder of sheet stacking to be avoided only by selecting an original image to be formed on the first side of the sheet.