In a continuous paper duplex printer, during duplex printing, a printer controller provides image data for odd-numbered and even numbered pages to front surface and back surface image processing apparatuses respectively, and a printing engine prints an image of an odd-numbered page on the front of continuous paper and parallelly forms an image of an even-numbered page different from an even-numbered page subsequent to the odd-numbered page on the back of the front of the continuous paper based on the image data processed by the processing apparatuses. In the continuous paper duplex printer, during single sided printing, the printer controller provides image data for odd-numbered and even numbered pages to the processing apparatuses, but when the image data is provided to one of the processing apparatuses, blank page image data or a blank page instruction is provided to another one of the processing apparatuses.