PURPOSE: To enable to effectively suppress the spurious by transversal wave mode, by giving slope or roundness to the side surface along the surface wave delivery path between the transducers of piezoelectric substrate.

CONSTITUTION: On the surface of the piezoelectric substrate 11, the transducer 12 converting electric signal into surface acoustic waves and the transducer 13 converting the surface acoustic waves into electric signal are provided opposingly with a given distance. Slope or roundness is formed along the surface acoustic wave delivery path between the transducers 12 and 13 on the substrate 11. With this constitution, the transversal wave mode having displacement component close to parallelism on the surface of the substrate 11 as the bulk wave of the surface acoustic waves is reflected toward different direction at the slope of the side surface of the substrate 11 to produce the mode conversion. Further, the surface acoustic wave toward the side surface of the substrate 11 is reflected toward different direction at the slope due to diffraction efficiency. Thus, the spurious by transversal wave mode can effectively be restricted.