PURPOSE: To apply polarization processing without causing destruction or crack of the substrate by interposing a dielectric material with lower dielectric constant than that of the substrate between an end ridge of a polarization electrode and the piezoelectric substrate.

CONSTITUTION: Dielectric rings 24a, 24b are formed to both sides of a piezoelectric substrate. The rings 24a, 24b are made of an epoxy group resin added with powder of a high dielectric constant and the dielectric constant is lower than that of the substrate 10. Then resist ink is formed except a position corresponding to a nonpolarized part 22 on the electrode film formed on both sides of the substrate 10 and only the electrode film on the part 22 is removed through etching and polarization electrodes 26a, 26b are formed except the part 22 and end ridges 28a, 28b are positioned on the rings 24a, 24b. A DC voltage is applied between the electrodes 26a and 26b to apply polarization processing only to the part overlapped with the polarization parts 26a, 26b. As a result, concentration of an electric field onto the end ridges 28a, 28b is avoided and the concentration of electrostriction is prevented.