A duplex printing apparatus capable of printing by performing a slow-up operation on the printing speed so that it increases from a low speed to a set speed at the time of duplex printing. Control means controls, on the basis of a drum rotation position signal and a drum speed signal from a home position sensor and an unshown encoder sensor, a sheet feeding unit, printing unit, sheet discharge unit, sheet re-feeding means and switching member, such that the printing speed (drum speed) slows up from a low speed to a set speed in each rotation of a plate cylinder, which corresponds to feeding of each sheet to the printing unit when duplex printing is started, and further controls a stepping motor of the sheet re-feeding means such that conveyance speed of a sheet receiving board synchronizes with the printing speed.