PURPOSE: To constantly perform suitable compensation of the opening of a throttle valve and enable highly precise and stable control compensating the motor driving period preset according to the difference between the set car speed and a current car speed, according to the potential level of a motor's power supply.

CONSTITUTION: A battery 1 serves as the power supply for a motor actuator 2 which is connected to a throttle valve 3 opened and closed under control of a control unit 4. The divided battery voltage (b) provided by dividing the voltage (a) of the battery 1 by means of potential dividing resistances 8 and 9 is applied to the input terminals of relevant comparators 5W7. The comparators 5W7 compares the voltage (b) with reference voltages (c) to (e) with each other and sends the results to the control unit 4. The control unit 4 compensates the driving period T preset according to the difference between the set car speed and a current car speed and stored in the motor actuator 2, on the basis of the output signals of the comparators 5W7.