PURPOSE: To apply a composite resonance mode and to obtain both a high voltage and a large output current by bringing a resonance mode in a lengthwise direction and a resonance mode in a lateral direction substantially into coincidence.

CONSTITUTION: A piezoelectric transformer is formed of a thickness direction polarized part and a lengthwise polarized part. The polarization is conducted by applying a high voltage in the thickness direction of a piezoelectric ceramic between the upper electrode 1a and the lower electrode 1b of the thickness direction polarized part. The polarization in the lengthwise direction is conducted by connecting the electrode 1a to the electrode 1b, using it as an earth side electrode, and applying a high voltage to the electrode 1c. The ratio of the amplitude in the lengthwise direction to that in the lateral direction is set to 0.1 to 0.7, and the ratio of the amplitude in the lengthwise direction to the thickness is set to 1 to 100. Thus, the resonance mode in the lengthwise direction to that in the lateral direction is brought into substantial coincidence, a composite resonance mode is applied, and a high voltage of 1 to 10kV and a large output current of 1 to 10mA can be realized at the same time.