An optical system scans an original document vertically and focusses an image of a linear portion of the document onto a photosensor array which comprises a large number of photosensor elements arranged in a line. The photosensor elements are sequentially enabled always in one direction to effect horizontal scan of the linear portion of the document. The optical system alternately scans up and down to maximize the vertical scanning speed. Reversal means control the optical system and photosensor array to effect rightward and leftward horizontal scan when the vertical scan direction is upward and downward respectively. The optical system may comprise two imaging systems to project erect and inverted images on the photosensor array respectively, the reversal means comprising a shutter for selectively unblocking the imaging systems. Alternatively, the output signals of the photosensor elements may be stored in a bi-directional shift register or a random access memory with a bi-directional address counter, and read out in a direction corresponding to the direction of vertical scan.