PURPOSE: To operate a duplex unit to be selected to the speed suited to each of modes, as well as to operate each mode so efficiently, by constituting it so as to vary the travel speed of a jogger fence with two conveying modes.

CONSTITUTION: A duplex unit 7 is controlled by a microcomputer CPU 70, while it performs the setting of start, stop and paper feed start modes and so on with a command out of a system controller 71. A jogger motor 61 is connected to an output board of the CPU 70 via a driver 72, and it alters the setting value of a timer or the like inside the CPU 70, whereby rotational frequency is easily alterable. And, speed of the jogger motor 61 is selected to the speed suited to a paper conveying mode, and at the time of a through mode, a jogger fence is moved without reducing a print speed and at the time of a stack mode, this jogger fence is moved with sufficient force even at time of stacking many sheets of paper, respectively. Accordingly, each mode is efficiently operatable.