A spoiler includes an outer member and an inner member. The outer member includes a base plate portion, a cover plate portion facing the base plate portion, and a bent portion connecting the base plate portion and the cover plate portion. The inner member is arranged between the base plate portion and the cover plate portion. The inner member includes a back plate portion facing the base plate portion, and a wall portion extending from the back plate portion toward the cover plate portion. The base plate portion and the back plate portion are vibration-welded, and the cover plate portion and the wall portion are vibration-welded. The wall portion includes a plurality of grooves forming a corrugated structure in a cross section taken along a plane perpendicular to a direction in which the wall portion extends from the back plate portion toward the cover plate portion.