A device (21) for reducing the air resistance of a tractor (11) transporting a baggage box (12). The reducing device is fixed to the cabin roof (15) of the tractor. The reducing device comprises a movable plate (22) supported movably with respect to the cabin roof, and a cylinder (23) disposed in the rear of the movable plate in order to alter the inclination angle of the movable plate. Height at the forward end of the movable plate is altered depending on the inclination angle of the movable plate. When the tractor travels while hauling a trailer (13), forward end of the movable plate is located at a first height (H1). When the tractor travels singly, forward end of the movable plate is located at a second height (H2) lower than the first height (H1).