The image read-out device comprises an integrally formed scanning optical unit in which a light source which emits light toward a document, a plurality of reflective mirrors reflecting the light which is emitted from the light source to the document and then reflected, an image forming lens converging the light reflected by a final reflective mirror which finally reflects the light, among the plurality of mirrors, and a light receiving sensor for receiving the light converged by the image forming lens are integrally mounted on a housing, the integrally formed scanning optical unit moving in a scanning direction and reading out an image of the document, wherein an opening is provided on each of top and bottom surfaces of the housing, and wherein the area of the opening provided on the bottom surface is equal or more than that of the opening provided on the top surface.