A printing apparatus which performs digital printing on one or both surfaces of a sheet, comprising: a sheet transfer unit configured to hold sheets and transfer the sheets one by one continuously; a printing unit configured to perform printing on the sheet transferred by the sheet transfer unit; a sheet supply unit configured to supply the sheets one by one to the sheet transfer unit; front lays provided downstream of the sheet supply unit in a transfer direction and against which a leading edge of the sheet abuts; and speed switching means, for the sheet supply unit, for switching a speed of the sheet supply unit between a case in which printing is performed on one surface of the sheet and a case in which printing is performed on both surfaces of the sheet, wherein when printing is performed on one surface of the sheet, the sheet supply unit is configured to be driven at a first supply speed at which the sheets are supplied to the sheet transfer unit one by one continuously, and the sheet transfer unit is driven at a transfer speed at which the sheets supplied from the sheet supply unit one by one continuously are transferred, characterized in that when printing is performed on both surfaces of the sheet, the sheet transfer unit is configured to be driven at the transfer speed and the sheet supply unit is driven at a second supply speed which is 1/2 the first supply speed, the sheet supply unit includes a stream feeder configured to transfer a plurality of sheets in a state in which the sheets partially overlap each other in the transfer direction, and the stream feeder includes a belt including a plurality of through holes and configured to move while the plurality of sheets are placed on the belt in a state where width of overlapping portions of the sheets in a case where printing is performed on one surface of the sheet is equal to that in a case where printing is performed on both surfaces of the sheet, and an air suction device sucking air via the plurality of through holes. a sheet transfer unit configured to hold sheets and transfer the sheets one by one continuously; a printing unit configured to perform printing on the sheet transferred by the sheet transfer unit; a sheet supply unit configured to supply the sheets one by one to the sheet transfer unit; front lays provided downstream of the sheet supply unit in a transfer direction and against which a leading edge of the sheet abuts; and speed switching means, for the sheet supply unit, for switching a speed of the sheet supply unit between a case in which printing is performed on one surface of the sheet and a case in which printing is performed on both surfaces of the sheet, wherein when printing is performed on one surface of the sheet, the sheet supply unit is configured to be driven at a first supply speed at which the sheets are supplied to the sheet transfer unit one by one continuously, and the sheet transfer unit is driven at a transfer speed at which the sheets supplied from the sheet supply unit one by one continuously are transferred, characterized in that when printing is performed on both surfaces of the sheet, the sheet transfer unit is configured to be driven at the transfer speed and the sheet supply unit is driven at a second supply speed which is 1/2 the first supply speed, the sheet supply unit includes a stream feeder configured to transfer a plurality of sheets in a state in which the sheets partially overlap each other in the transfer direction, and the stream feeder includes a belt including a plurality of through holes and configured to move while the plurality of sheets are placed on the belt in a state where width of overlapping portions of the sheets in a case where printing is performed on one surface of the sheet is equal to that in a case where printing is performed on both surfaces of the sheet, and an air suction device sucking air via the plurality of through holes.