A duplexer (3) for connecting a transmitter (1) and a receiver (2) to a common receiving/transmitting antenna (4) is constructed from an isolator (5) and a receiving band-pass filter (6). The receiving band-pass filter (6) prevents a transmitted wave from entering the receiver (2) and the isolator permits signals to pass only in the direction of transmitter (1) to antenna (4). Impedance matching is effected so that the isolator can prevent a received wave from entering the transmitter while suppressing the entrance of signals radiated from other mobile units in the same transmitting frequency band but at a different operating frequency. <IMAGE>
