PURPOSE: To improve the driving efficiency by providing a feedback loop controlling an effective value of an AC component of a superimposing signal between an output current of a piezoelectric vibrator drive amplifier circuit and a current flowing to the piezoelectric vibrator to a prescribed value.

CONSTITUTION: A power voltage Vcc is fed to a driving power amplifier circuit 7 comprising a voltage controlled oscillation circuit 5, an amplifier circuit 6, an output transformer 8 and an output transistor (TR) Q3 and the frequency of the oscillation circuit 5 drives a piezoelectric element TD. A mechanical structure 1 is incorporated with the piezoelectric element TD to constitute the piezoelectric vibrator 2 as a whole. An output current Ie of the amplifier circuit 7 and a current Io flowing to the peizoelectric element TD are fed to a current transformer CT of a detection control circuit 9, from which a control voltage S of the oscillation circuit 5 is obtained via a tuning transformer 10, a rectifier smoothing circuit 14 and a differential amplifier 13. Then the oscillation frequency of the oscillation circuit 5 is controlled so as to make the effective value of the AC component of the superimposing signal between the current Ie and the current Io as a constant value.