PURPOSE: To lift and lower an air balance panel at low and high speed running time, prevent the damage of the same panel, and improve the running safety by operating an actuator through a vacuum switchover valve by means of an air speed sensor.

CONSTITUTION: When a diaphragm 11 that constitutes a wind pressure sensor 7 by the wind pressure following the high speed running is moved against a spring 12, a rod 13 is shifted and a contact 14 is made to contact. Thus, an electric current is applied to the lead wire 15 of a vacuum switchover valve 8 and a solenoid valve 16 is excited and then a plunger 17 is attracted. As a result, a valve 18 is opened and the vacuum is applied to an outlet 20 from an inlet 19 communicated to a manifold 6 and then to an actuator 2. Consequently, the rod of the actuator 2 is lowered and an air balance panel 1 connected to the lower end 4 swings centering a fulcrum 30 and is lowered. On the other hand, at a low speed running, the diaphragm 11 is restored by the spring 12 and the above- mentioned panel 1 is raised.