To reduce the yawing moment and air resistance that act on a vehicle receiving cross wind and the air resistance that acts on the vehicle receiving no cross wind in a well-balanced way.

A cavity spoiler 10 comprises a roof spoiler part 12 that extends back and down from the rear edge of a roof 4 at the first preset angle with the imaginary extension plane of the roof 4, and side spoiler parts 14 that extend back and inward from the rear edge of body sides 6 at the second preset angle with the imaginary extension plane of the body sides 6. The roof spoiler part 12 chiefly offers an air-resistance reducing function when the vehicle receives no cross wind, while the side spoiler parts 14 chiefly offer an air- resistance reducing function when the vehicle receives the cross wind. The first preset angle is fixed not to cause the roof spoiler part 12 to act as an air-resistance increasing factor in cross wind.