An image reading apparatus comprising: a document platen on which an original document is placed; a sensor unit extending in a first direction and configured to read the original document while moving in a second direction orthogonal to the first direction; a belt to which the sensor unit is attached; a first driven pulley and a second driven pulley provided at a distance from each other in the second direction, wherein both of the first driven pulley and the second driven pulley are in contact with an inner surface of the belt; a drive pulley provided between the first driven pulley and the second driven pulley in the second direction, wherein the drive pulley is in contact with the inner surface of the belt; a motor fixed to the apparatus and configured to drive the drive pulley; and a third driven pulley provided between the first driven pulley and the second driven pulley in the second direction, wherein the belt is bent at a position of contact of the third driven pulley with an outer surface of the belt; characterized in that the belt is wound around the first driven pulley, the drive pulley, the third driven pulley, and the second driven pulley in this order, wherein, in the first direction, a rotation center of the third driven pulley is nearer from a rotation center of the first driven pulley than a rotation center of the drive pulley, and wherein, in the second direction, a distance between the rotation center of the third driven pulley and the rotation center of the first driven pulley is not greater than a distance between the rotation center of the drive pulley and the rotation center of the first driven pulley. a document platen on which an original document is placed; a sensor unit extending in a first direction and configured to read the original document while moving in a second direction orthogonal to the first direction; a belt to which the sensor unit is attached; a first driven pulley and a second driven pulley provided at a distance from each other in the second direction, wherein both of the first driven pulley and the second driven pulley are in contact with an inner surface of the belt; a drive pulley provided between the first driven pulley and the second driven pulley in the second direction, wherein the drive pulley is in contact with the inner surface of the belt; a motor fixed to the apparatus and configured to drive the drive pulley; and a third driven pulley provided between the first driven pulley and the second driven pulley in the second direction, wherein the belt is bent at a position of contact of the third driven pulley with an outer surface of the belt; characterized in that the belt is wound around the first driven pulley, the drive pulley, the third driven pulley, and the second driven pulley in this order, wherein, in the first direction, a rotation center of the third driven pulley is nearer from a rotation center of the first driven pulley than a rotation center of the drive pulley, and wherein, in the second direction, a distance between the rotation center of the third driven pulley and the rotation center of the first driven pulley is not greater than a distance between the rotation center of the drive pulley and the rotation center of the first driven pulley.