To improve throughput and cut electric power consumption during perfecting by detecting the length of paper supplied by a paper supply means and controlling paper reversing time of a paper reversing means.

When blank paper is supplied by a paper supply means 10 at a rate taking into account the paper reversing time of a paper reversing means 13, the length of the blank paper is detected by a paper length detecting means 11 and a detection signal is outputted to a reversing time control means 15. The blank paper is then delivered to a printing means 12 and subjected to single-sided or two-sided printing. At this time, the reversing time control means 15 outputs a control signal corresponding to the length of paper received. The paper, after printing on one side thereof, is reversed on the basis of the control signal from the reversing time control means 15, and supplied to the printing means 12 by the paper re-feed means 14 for printing on the other side.