A power source 1 and an operating circuit 4 incorporating a waveform generating circuit 5 for synthesizing composite wave W obtained by synthesizing a sine wave of primary oscillations and a sine wave of secondary oscillations of a Rosen-type piezoelectric transformer and an amplifying circuit 6 raises the voltage of the composite wave so that composite wave W' (having a waveform similar to that of W) is, as input voltage V.sub.in, supplied to the Rosen-type piezoelectric transformer PT. Thus, output voltage V.sub.out having greater output electric power and higher transmitting efficiencies as compared with output voltage which can be obtained from a conventional structure in which input voltage in a single oscillation mode is employed can be produced to load resistor R.sub.o.