An insulating film including an aluminum nitride film is provided on a support substrate to be supported thereby. Then, a lower electrode, a piezoelectric thin film, and an upper electrode are provided in that order on the aluminum nitride film. The piezoelectric film is disposed between the lower electrode and the upper electrode which oppose each other. Furthermore, the aluminum nitride film is subjected to a plasma treatment in an oxygen-containing atmosphere to form an oxide layer on the aluminum nitride film, the oxide layer being made smoother than the aluminum nitride film.