A duplexer includes first and second filter circuits and first and second wirings. The first filter circuit allows a signal of a first frequency band to pass therethrough between a first terminal and a common terminal and includes a first resonator which is connected at one end to a line disposed between the first terminal and the common terminal to branch off from the line. The second filter circuit allows a signal of a second frequency band, which is different from the first frequency band, to pass therethrough between a second terminal and the common terminal. The first wiring is connected at one end to the common terminal and is opened at the other end. The second wiring is connected at one end to the other end of the first resonator and is grounded at the other end. The first wiring is electromagnetically coupled with second wiring.