PURPOSE: To reproduce a sound signal easy to hear by adding a circuit to com press the amplitude of a transmitting sound signal to a transmitting side and adding a circuit to expand the amplitude of a signal extracted by an adaptive echo canceler to a receiving side.

CONSTITUTION: The transmitting side is equipped with a compressing circuit 2 to compress the amplitude of the sound signal and to output a compressed sound signal and a noise generating circuit 3 to output a noise signal in a fre quency band occupied by the compressed sound signal, and a secret call signal into which the compressed sound signal and the noise signal are added is trans mitted to a transmission path 10. The secret signal is guided from a receiving side input terminal 11 to the input terminal of an adaptive echo canceler 13. On the other hand, the output signal of a reference noise generating circuit 12 is supplied to the other input terminal of the adaptive echo canceler 13 as a reference noise signal, the amplitude of the signal extracted by the adaptive echo canceler 13 is expanded by an expanding circuit 14 and outputted from a receiving side output terminal 15. Thus, a reproducing sound offensive to the ear in hearing can be eliminated.