Reference images on the respective surfaces of a reference document transported on a transportation path are read out by a first image reading section and a second image reading section, respectively. As a result a first reference readout image and a second reference readout image are acquired. in accordance with the difference between the changes in the sub-scanning direction in the first reference readout image and the changes in the sub-scanning direction in the second reference readout image, the reading cycle of the first image reading section X 1 and the reading cycle of the second image reading section X 2 are set. This makes it possible to equalize the magnifications of the respective images on the front and back sides of the document, even if the speeds of the document at the times of passing through readout positions for the front and back sides change over time, on account of the wear of a transportation roller.