Image reading device and method of scaling up or down image to be read

Mirror driving unit (21, 22), the document is changed from (K) to the lens (16) optical path length, changing the magnification of an image on the image sensor (17) for imaging, where, for example, a higher density reading to read take the case of a narrow range, the small size of the document to guide the lens (16 by reducing the reflected light from the document number (K) in reaching the lens (16) until the mirror (12, 13) in ) manner, that a given mirror (12, 13) is rotated so as not via the mirror (14), to shorten the optical path length, so that the image on the image sensor (17) to enlarge the image. Accordingly, without increasing the number of pixels read and reducing the cost to obtain sufficient accuracy in the image as needed.