A spoiler includes an upper member and a lower member. The upper and lower members are joined together by vibration welding. One of the upper and lower members includes a main welding rib and an auxiliary welding rib. Each of the main welding rib and the auxiliary welding rib is protruded toward the other of the upper and members so as to be vibration welded to the other of the upper and lower members. The main welding rib is arranged to extend along a vibration direction of the vibration welding, and the auxiliary welding rib is arranged to extend along a direction perpendicular to the vibration direction. A height of the auxiliary welding rib is less than a height of the main welding rib before the vibration welding. The auxiliary welding rib is spaced apart from the main welding rib.