PURPOSE: To prevent deterioration in etching rate by covering the reverse surface of a piezoelectric substrate with aluminum foil and performing reactive ion etching.

CONSTITUTION: The reverse surface of the piezoelectric substrate 1 is covered with the thin-film aluminum foil 2 which is cut a little bit larger than the piezoelectric substrate 1. Then reactive ion etching is carried out on the surface of the piezo-electric substrate 1. Thus, a charge given to the etched surface of the piezoelectric substrate 1 is held at the same potential with the reverse surface of the piezoelectric substrate 1 through the aluminum foil 2, so electrostatic effect which causes deterioration in etching rate is eliminated. Thus, the deterioration in etching rate is prevented.