An output signal of an optical reading line sensor is converted into a digital signal by an A/D converter and, thereafter, is shading corrected by a shading correction circuit, and is inputted to a magnification processing circuit through a selector. An optical reading scanning speed by the line sensor can be freely changed. There are a first mode for zooming a read image by changing the scanning speed of the line sensor, a second mode for zooming the read image by the magnification processing circuit, and a third mode for commonly performing the zoom by changing the scanning speed of the line sensor and the zoom by the magnification processing circuit. Any one of the modes is adaptively selected in accordance with a size of original and a reading resolution.