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, and the terminal voltage of the backup capacitor is adjusted so as to become equal to the calculated required charging voltage.