PURPOSE: To increase the energy conversion efficiency by providing the roller-screen type electrode in the piezoelectric film.

CONSTITUTION: When the signal is applied between input terminals 5A and 5B from AC signal source 6, the surface wave decided by the size of the roller-screen type electrode and the thickness of the piezoelectric film propagates between transducers 15 and 16. In this case, the electric line of force produced between roller- screen type electrodes 11A and 11B propagates largely in piezoelectric film 14. Thus most of the electric line of force can contribute to the oscillation of the surface wave, and as a result the conversion efficiency can be increased between the electricity and the elastic surface wave.