A second substrate 21 composed of a material having a lower dielectric
constant than that of a piezoelectric substrate 2 having a
transmission-side filter region 12 and a receiving-side filter region 13
formed therein is joined to the other main surface of the piezoelectric
substrate 2, and a conductor layer 22 is formed throughout the other main
surface of the second substrate 21. The effective dielectric constant of
the substrate is reduced, thereby making it possible to reduce a
parasitic capacitance formed between an input electrode section 5 in the
transmission-side filter region 12 and an output electrode section 6 in
the receiving-side filter region 13 and to improve isolation
characteristics.