Patent ID: 8857974

Claim:
A duplex printing apparatus for printing, with a single digital printer, on both surfaces of a web of paper for rotary printing traveling over between a paper supply unit and a paper takeup unit, characterized in that it comprises: a pair of feed roll units arranged across the digital printer from each other in a direction of travel of the web of paper and each adapted to be driven synchronously with the paper supply and takeup units for feeding the web of paper to travel forwards and backwards while an upper surface of the web of paper faces an under surface of the digital printer; a turnover unit arranged at one of a site between the paper supply unit and the feed roll unit on the paper supply side and a site between the paper takeup unit and the feed roll unit on the paper takeup side, whereby the web of paper is turned over through, and is allowed to bypass, a turnover path of the turnover unit; a mark sensor arranged on at least one of sides across the digital printer from each other in a direction of travel of the web of paper and upstream in a direction of travel of the web of paper, an upper surface of which is being printed on by the digital printer, for detecting a timing mark which has been printed by the digital printer on an under surface of the web of paper; and a control unit operable in response to a detection value from the mark sensor for controlling the digital printer so that on the upper surface of the web of paper there is printed an image corresponding to an image that has been printed on the under surface of the web of paper.