the same or similar components shown in fig7 and the explanations thereof are omitted in fig1 reference numeral 5 indicates a bimorph which is attached on a mounting portion of an elastic member 2 positioned on a rear surface of a supporting plate 1 reference numeral 6 represents a resistor both ends of which are connected to electrodes 5a and 5b of the bimorph 5 fig2 schematically illustrates a construction of the bimorph 5 and resistor 6 employed in the preferred embodiment as shown in fig2 when external force 34 f 34 is applied to the bimorph 5 this bimorph 5 is bent toward the application direction of the external force as a result an electric charge is produced between the electrodes 5a and 5b of the bimorph 5 thereafter when the application of this external force 34 f 34 is interrupted residual vibrations are produced at the bimorph 5 as a result an alternating voltage is produced between the electrodes 5a and 5b of the bimorph 5 in response to the residual vibrations when the electric charge generated between the electrodes 5a and 5b is discharaged through the resistor 6 and a joule heat is produced at the resistor 6 at this.