An antenna duplexer includes a piezoelectric substrate having a surface, a grounding terminal to be grounded, first and second surface acoustic wave (SAW) filters mounted on the piezoelectric substrate, first and second terminals connected to the first and second SAW filters, respectively, and a line mounted on the surface of the piezoelectric substrate between the first SAW filter and the second SAW filter. A stray coupling path is produced between the first and second terminals. A first end of the line is connected to the grounding terminal. A second end of the line opens and is coupled capacitively to the stray coupling path. The antenna duplexer has a large isolation characteristic between the surface acoustic wave filters.