FIG. 3 explains a voltage difference generated in the piezoelectric element when the acceleration F is applied thereto; In FIGS. 4A to 4C, FIG. 4A is a top view of a supporting member integrally molded with a metal frame, FIG. 4B is a cross-sectional view of the supporting member taken along the line I--I of FIG. 4A, and FIG. 4C is a cross-sectional view of the supporting member taken along the li