The first image which is a continuous image divided into pages is printed on the first surface of web by the first printer, and the start mark is printed at the head of each page. The second image which is a continuous image is printed on the second surface by the second printer in synchronization with recording synchronization pulses. In printing of the second image, obtained is a difference between the number of pulses inputted to the second printing mechanism between detections of preceding and succeeding start marks, and the number of pulses estimated to be inputted thereto in the case where expansion or shrinkage of web doesn't occur, to adjust the frequency of pulses on the basis of the difference. Therefore, the second image is printed in accordance with the first image on the web without being affected by expansion or shrinkage of web.