A substrate has a first thermal expansion coefficient and a piezoelectric thin film has a second thermal expansion coefficient. The piezoelectric thin film is mainly composed of a potassium sodium niobate (K,Na)NbO.sub.3 with a perovskite structure. A curvature radius of a warping of the substrate provided with the piezoelectric thin film due to difference between the first and the second thermal expansion coefficients is 10 m or more at room temperature.