In an alternate energization process for alternately energizing a valve closing electromagnet and a valve opening electromagnet periodically, one of the valve opening electromagnet and the valve closing electromagnet is de-energized and thereafter the other electromagnet is de-energized at a time delayed by a predetermined time Toff. The alternate energization process is finished when the displaced position of a valve head which is detected by a displacement sensor has reached a predetermined position Vinit while the valve opening electromagnet is being energized. Thereafter, the valve opening electromagnet is continuously energized to seat the valve head in a closed position (initial position) from the time when the displaced position of the valve head has reached a predetermined position Vstart.