1. An image reader having a main scanning direction and a sub-scanning direction perpendicular to the main scanning direction, comprising: a platen unit that includes a document area where a document is placed and a reference pattern area on an outer periphery of the document area, wherein the reference pattern area includes reference patterns drawn at predetermined intervals in a sub-scanning direction; a scanning unit that optically scans the document area and the reference pattern area and forms an image on an imaging device via a lens, from which the imaging device generates an electrical signal; an image data generation unit that generates image data from the electrical signal generated by the imaging device and stores the image data in an image memory; a drive unit that reciprocates the scanning unit in the sub-scanning direction; an image processing unit that performs predetermined image processing on the image data stored in the image memory; a scaling factor calculation unit that calculates correction scaling factors for the read images in the sub-scanning direction by comparing a stored interval of the reference patterns with an interval of the read images of the reference patterns; and a control unit that uses the correction scaling factors to adjust magnification accuracy.