A piezoelectric transformer (21) comprises a piezoelectric rectangular body (23) equally divided into first to fourth regions (A1, A2, A3, and A4) in a longitudinal direction. The first and the fourth regions (A1, A4) are provided with plane electrodes (24, 25, 26, and 27) as input electrodes and polarized in a thickness direction. Strip-like electrodes (30, 31) are formed at the boundary between the remaining second and third regions (A2, A3) as output electrodes. An AC input voltage having a frequency corresponding to a wavelength equal to a half of the length of the piezoelectric rectangular body is applied to the input electrodes to vibrate the piezoelectric plate in a two-wavelength resonance mode. A voltage produced by a piezoelectric effect in response to the vibration is extracted from an output terminal. The transformer is small in size, low in vibration rate and heat generation, and excellent in efficiency. As a modification, the input electrodes may be formed in two or three regions which are opposite to each other in displacement distribution in the two-wavelength resonance without being restricted to the first and the fourth regions while the output electrode is provided in another region.