An operator sets a distance from the page head of an alignment mark using a mark position setting switch mounted in a duplex printing system. A first printer forms the alignment mark in a position of the distance from the page head of a web, and reports position information about the alignment mark to a second printer. The alignment mark is detected by mark detection means of the second printer. Control means performs control of a web transport speed so that a phase between generation timing of a mark detection signal which the mark detection means generates by detecting the alignment mark and control timing calculated from alignment mark position data, alignment mark interval data and a web feed control signal generated at a preset period becomes constant.