The duplexer includes a transmit filter connected between an antenna terminal and a transmit terminal, and a splitter circuit and a receive filter connected in series between the antenna terminal and the receive terminal. The splitter circuit includes at least one phaseline connected between the antenna terminal and the receive filter and at least one resonator connected in parallel with the phaseline. According to the present invention, the length of the phaseline can be made shorter, so the entire duplexer can be made more compact in size. In addition, an inductors are preferably connected between each ends of the phaseline and the resonator. In this case, the inductance of a wire or via formed within the package can be used.