To provide an image reader in which deterioration in the aspect ratio of an image is prevented even when a read magnification is deviated from a set value due to a machine error and an image with high accuracy is easily obtained.

In the image reader, a reference mark A and a center mark C of a line pattern 3a of a reference pattern are projected onto a CCD 26 through a zoom lens 25 and an actual projection magnification of the zoom lens 25 is detected by addresses of pixels a, c corresponding to the projected image. Then a subscanning speed calculation section calculates a subscanning speed in response to the detected actual projection magnification to drive a subscanning motor. Thus, the subscanning speed in response to the actual projection magnification is set independently of presence of a magnification error due to a mechanical error in the case of zooming by the lens and the accuracy of the image quality is warranted.