A most recent electrostatic capacitance value for a backup capacitor is measured periodically. Each time the most recent electrostatic capacitance value is measured, a charging voltage (a required charging voltage) that is required in order to cause a return operation of a valve from the setting opening at that time to an emergency opening/closing position (for example, the fully closed position) is calculated based on the electrostatic capacitance value that has been measured. If the required charging voltage that has been calculated is higher than the terminal voltage that has been detected, then a value for a charging current value is determined based on the actual degree of opening and the setting opening of the valve, and the backup capacitor is charged until the terminal voltage reaches the required charging voltage.