Reference patterns disposed on a platen unit are read by optically scanning the reference patterns and stored in an image memory as pattern image data. An interval between the reference patterns, the interval being stored in advance, is compared with intervals between read images of the reference patterns to calculate correction scaling factors for scaling the intervals between the read images. Document image data read by scanning a document placed on the platen unit is multiplied by the calculated correction scaling factors to adjust magnification accuracy of an image of the document.