The present invention is a piezoelectric actuator unit 29 having piezoelectric vibrators 33 vibrating in the lamination direction of electrode layers and piezoelectric material layers which are alternately laminated. The second end portion of the braided aluminum lead wire is joined to the magnet wire after the step of brazing. The hollow particles may consist essentially of silica. A revised actual current IACTUAL\u20142 is obtained as a function of transition times with CMAX\u20142. Alternatively, a knife blade or the like may be provided adjacent the vent panel to effect severing thereof under a predetermined overpressure in lieu of a line of weakness in the panel.