PURPOSE: To assure safety in case of faults of a sensor or an actuator by closing a throttle valve when the opening of the throttle valve reaches the maximum value in slow driving or when the slipping of a driving a wheel is detected, and by cancelling constant speed driving control by means of the closing of the throttle valve when a brake panel is used in a constant speed driving mode.

CONSTITUTION: When there is a fault in a car speed sensor 8 in a slow driving mode and the speed far slower than the actual car speed is output, the excessive opening of a throttle valve 6 over the maximum value is prevented, while if there is a fault in a controller 11 and the throttle valve 6 is opened to an extraordinary level, the throttle valve 6 is restored to its totally enclosed position by a limiter 21. Based on the signals from a sensor for the space between cars 9 and from an obstacle sensor 20, the alarm is given by an alarm display device 10. Further, when the slipping of a driving wheel is detected by a sensor, the throttle valve 6 is closed by the controller 11. Through the actuation of a brake switch 27 in a constant speed driving mode, feeding to a throttle actuator 5 is shut off, and a constant speed driving mode is thus cancelled.