A duplexer is provided, which includes a first, a second and a third signal ports; a first filter and a second filter. The first filter has first, second, and third resonant circuits that have first, second and third inductors, respectively. The first, second and third inductors are mutually inductive. The first and third resonant circuits are electrically connected to the first and second signal ports, respectively. The second filter has fourth, fifth and sixth resonant circuits that have fourth, fifth and sixth inductors, respectively. The fourth resonant circuit is connected in series with the first resonant circuit. The fifth inductor and the fourth inductor are mutually inductive. The sixth resonant circuit is electrically connected to the third signal port. The second filter further has a main capacitor connected in series with the fifth and sixth resonant circuits respectively and located therebetween.