PURPOSE: To avoid deterioration of the deceleration performance of a car by suspending the control for the return speed of a throttle valve by a dashpot by feeding a negative pressure when a controller is operated in deceleration, thus preventing delay or return of the throttle valve.

CONSTITUTION: On control, it is judged whether the car speed detected by a car- speed sensor 23 is above a predetermined value or not. When the car speed is above the predetermined value, it is judged whether the controller is in operation or not, by the output signal of a brake switch 22, and if the controller is in operation, the signal for closing a switch 20 is outputted. A solenoid 12 is put into conduction by closing the switch 20, and the port (a) of a negative-pressure selector valve 11 is connected to a port (c) to introduce the negative pressure in an intake pipe into a dashpot chamber 6. Therefore, a diaphragm 5 and a rod 7 are moved rightward to prevent a throttle lever 8 from being attached onto the rod 7, and return of a throttle valve 2 is accelerated.