In printing both sides of plural sheets of paper by using a digital copy machine having a circulating-type duplexing unit, when a back face of the first page is printed after printing a front face thereof, if it is likely to collide with a sheet of paper for the second page, priority is given to printing of the back face of the first page. If the front faces of both first and second pages are already printed, then control proceeds so as not to collide with the sheet of paper for the back face of the second page irrespective of delay in printing the back face of the first page.