An image reading device has an optical unit, a pair of rail portions, and a driving device. The optical unit reads an image on the document. The pair of rail portions support the optical unit such that this is slidable in the sub scanning direction. The driving device enables the optical unit to move in the sub scanning direction. The driving device engages with the optical unit substantially at the position of its center of gravity in the main scanning direction, and the pair of rail portions are arranged one on each of one and the other sides in the main scanning direction so as to be equidistant from the position of the center of gravity.