PURPOSE: To get stable braking effect by making a braking board project from a car body by a fixed amount according to a signal to press on a brake pedal when a car is running at more than medium speed.

CONSTITUTION: A transparent braking board 4 is installed to be able to project from a fixed place of a car body rear part 1 against an air current flowing on a car body when the car is running. A drive mechanism 5 displaces the braking board 4 from the body by making the board project by a fixed amount and at a fixed speed or recovered to the body side. An automatic controller 6 has such a structure as receiving a through a wiring 24, brake pedal pressing signal transmitted from a sensor 23 which senses pressing motion of the brake pedal 22 and the operation of the drive mechanism 5 is controlled in such a way that the braking board 4 projects from the body only at medium speed (about 60km p.h.) or more and the amount of projection is set to a proper value according to the car speed. Thus stable braking effect can be obtained.