There is provided an image reading device capable of preventing disorder of an aspect ratio of an image even when a reading magnification ratio deviates from a set value due to a mechanical error and easily obtaining an accurate image. In this image reading device, a reference mark and a center mark of a line pattern of a reference pattern are projected on a CCD by a zoom lens, and an actual projection magnification ratio of the zoom lens is detected by use of addresses of pixels corresponding to a projection image. Then, a sub-scanning speed calculating section calculates a sub-scanning speed corresponding to the detected actual projection magnification ratio and drives a sub-scanning motor. Therefore, the sub-scanning speed corresponding to the actual projection magnification ratio is set regardless of the presence or absence of a magnification ratio error attributed to a mechanical error in a zooming stage on the lens side, thereby allowing an image accuracy to be guaranteed.