A printer, including a printing unit to print print data on print medium. The printer includes a memory to temporarily store the print data supplied from a user terminal, a controller to temporarily cancel a duplex printing mode of the printer and to control the printing unit to print the print data in a normal printing mode, when the duplex printing mode is preset and a quantity of the print data temporarily stored in the memory is within a range of predetermined quantities, a printing path along which the print medium is provided to the printing unit so as to have an image printed on a first side thereof, an ejecting path along which the pieces of print media, having had the image printed on the first sides thereof, are ejected from the printer in the normal printing mode, or are partially ejected from the printer in the duplex printing mode, and a reversing path to convey the print medium from the partly ejected position of the ejecting path and toward a beginning of the printing path to position the print medium to be conveyed along the printing path so as to have another image printed on a second side thereof.