To provide a duplexer which is small in size and excellent in separation characteristic of transmission/reception signals. An antenna port is disposed on a center of a rearward side in a disposition area of a duplexer, a high band side filter and a low band side filter are respectively disposed on a left side and a right side of the antenna port, a parallel arm at a last stage in the low band side filter, a parallel arm at a last stage in the high band side filter, a first signal port and a second signal port seen from the antenna port are positioned on a forward side of the antenna port, a ground side of a parallel arm on a front stage side of the parallel arm at the last stage in the low band side filter and a ground side of a parallel arm on a front stage side of the parallel arm at the last stage in the high band side filter are mutually connected via a conductive path formed on a piezoelectric substrate at a rearward side of the antenna port, and the parallel arms connected by the conductive path are positioned on the rearward side of the parallel arms at the last stages. Accordingly, a separation characteristic at a high band side is improved.