Abstract: A sheet-size detecting mechanism for an image scanning apparatus detects the width of a sheet before a complete image of a sheet is acquired by the means of varying optical paths and obtaining a partial image of the sheet in advance so as to judge a width of the sheet. The mechanism includes a light source, a reflecting device having at least one reflecting mirror, a path-routing device, a photosensor, and a detecting position defined on a sheet transporting path of the sheet. The path-routing device may be rotatable, movable or fixed and reflects light rays from the detecting position and a scanning position. The path-routing device changes the optical path so that the partial image of the sheet at the detecting position may be acquired and the width of the sheet may be judged precisely. In addition, the manufacturing cost can be decreased because the sheet-size detecting mechanism and an image scan mechanism may have shared components.