A parent telephone set comprises a transformer, a sidetone level reducing circuit, and a first secret talk processing circuit for executing digital secret talk processing, a counterpart slave telephone set comprises a second secret talk processing circuit for executing digital secret talk processing, a delaying circuit for delaying an aural signal from the slave telephone set microphone by a specified delay time, and a level reducing circuit for reducing the level of output from the second secret talk processing circuit according to the output from the delaying circuit. The specified delay time in the delaying circuit is set according to the time required for digital secret talk processing in the first secret talk processing circuit as well as in the second secret talk processing circuit.