The other end of a hub portion is located farther from an attachment portion at a lateral portion, which is located in a lateral direction with respect to the vehicle, than at a vertical portion, which is located in a vertical direction with respect to the vehicle. The vertical portion of the hub portion has a concave end face on the side of the other end of the hub portion, which is formed to get closer to the attachment portion as the concave end face approaches a portion closest to the attachment portion substantially at the center of the hub portion in the lateral direction.