A diplexer includes a first bandpass filter provided between an input terminal and a first output terminal and selectively passing a signal in a first frequency band, and a second bandpass filter provided between the input terminal and a second output terminal and selectively passing a signal in a second frequency band. The first bandpass filter includes a plurality of first resonators. The second bandpass filter includes a second resonator and a series resonant circuit. The series resonant circuit is composed of a capacitor provided between the input terminal and the second resonator, and an inductance component of a line that connects the input terminal and the second resonator via the capacitor.