When a computer determines that an accelerator pedal and a brake pedal are simultaneously stepped based on the signals from the accelerator position sensor and a brake switch, the first switch is turned off so that the accelerator sensor signal which will be transmitted into the computer is compulsorily made zero. A throttle-position command value which will be transmitted from the computer into a throttle actuator is reduced to a value corresponding to the idling state. If an engine output is not reduced, the computer turns off a second switch so that a throttle actuator is de-energized. A throttle valve is compulsorily brought into a position corresponding to an idling state by a biasing force of a throttle return spring.