PURPOSE: To improve energy efficiency by providing a control means so that the frequency of a weight at one end of a piezoelectric element to be vibrated by inertia and that of the stretching movement of the element are matched with a resonance frequency.

CONSTITUTION: When a voltage is applied on the piezoelectric element 111 at a relatively high frequency, the element 111 is stretched. The weight 114 having a large mass is moved by inertia. When the voltage drops, the element 111 is contracted, and the element is moved by the inertia of the weight 114. When the frequency of the voltage applied on the element 111 is matched with the resonance point of the system, resonance is caused, and a large amplitude is obtained. The voltage VR imparting a reference frequency is transduced into a signal indicating a frequency through an adder 21. The signal is amplified by an amplifier 23 to drive the element 111. The vibrating state of the element is detected by a pickup 24. By this method, energy is saved.