and contracts in the radial direction it is not required that the fixation of the first piezoelectric substrate 31 is secured around the entire circumference thereof when the ac voltage is applied by means of the amplifier 51 between electrodes 37 and 34 of the first piezoelectric substrate 31 the expanding and contracting forces act in the radial direction since the circumference of one surface of substrate 31 is fixed the amount of flex is dependent on the expansion and contraction forces and to the surface area fixed along the circumference the resultant vibration of piezoelectric substrate 32 generates an ac voltage across the electrodes 37 and 46 this ac voltage leads the ac voltage applied on the second piezoelectric substrate 32 by 90 u00b0 mechanical resonant frequency determined by the piezoelectric vibrator consisting of the first and second piezoelectric substrates 31 and 32 and by added mass of a medium being in contact with the piezoelectric vibrator positive feedback via a positive feedback circuit 52 to the first piezoelectric substrate 31 compensates the foregoing phase lead whereby the piezoelectric vibrator consisting of the first and second piezoelectric substrates 31 and 32 performs oscillations at the resonant frequency referring to fig2 there is illustrated the principal elements of a piezoelectric vibrator the piezoelectric vibrator may have the same construction as that of the piezoelectric vibrator shown in fig1 but of course will.