A piezoelectric actuator is driven by continuously applying a pulse voltage or an AC voltage to a piezoelectric element, so as to cause the piezoelectric element to generate heat because of a dielectric loss of the piezoelectric element itself. If the temperature of the piezoelectric element reaches a temperature not less than 15.degree. C., moisture contained in atmosphere is effectively prevented from entering into a resin casing of the piezoelectric actuator enclosing therein the piezoelectric element. Otherwise, moisture condensation or sweating would occur on a surface of the piezoelectric element. Therefore, even if the piezoelectric actuator is used in atmosphere of high humidity, a discharge breakdown will not occur, and reliability is remarkably increased.