Abstract: A speed reduction gear including a semicircular control gear is provided coaxially with a motor shaft between a motor and a throttle valve. A connecting lever which has a valve opening contact and a valve closing contact is secured to a shaft of the throttle valve. When the control gear turns in the valve closing direction, it engages with the valve closing contact, and the throttle valve is closed. A safety stopper and its actuator are also provided. When the control gear turns in the valve opening direction while the stopper is driven by the actuator to extend in the path of the control gear, the control gear is prevented from further turning in the valve opening direction. When a cruise control switch is turned on, the stopper is retracted by the actuator from the passage and the gear turns in the valve opening direction until it abuts the valve opening contact of the connecting lever, which opens the throttle valve.