A piezoelectric transformer drive circuit of a full-bridge type having high efficiency minimizes the effect of voltage fluctuation of an input voltage V.sub.CC. The piezoelectric transformer drive circuit includes transistors that drive primary electrodes of a piezoelectric transformer, an inductor, a tube current detection circuit, a first error amplifier, a voltage controlled oscillator that outputs an oscillation clock and a triangular wave signal, an applied difference voltage generating circuit that outputs the difference between the voltages of the primary electrodes, an applied voltage detection circuit, a second error amplifier, a PWM comparator that compares this voltage with the triangular wave signal and outputs a PWM signal, a frequency divider that frequency-divides the oscillation clock, an AND circuit that controls the transistors by computing the AND of the output signal thereof and the PWM signal, and an AND circuit that controls the transistors by computing the AND of an inverted signal of the output signal of frequency divider and the PWM signal.