A duplexer according to the present invention includes a first port, a second port and a third port for external input/output, a first path formed between the first port and the third port, a second path formed between the second port and the third port, a phase shifting part provided for each path, and a resonating part provided for each path. At least any of the resonating parts has a ring conductor having a length equal to one wavelength at a resonant frequency or an integral multiple thereof, a plurality of passive circuits, and a plurality of switches each of which is connected to a different part of the ring conductor at one end and to any of the passive circuits at the other end. A switch may simply be connected to a ground conductor instead of being connected to the passive circuit.