PURPOSE: To improve the signal-to-noise ratio of a reproducing sound signal and to improve secrecy on a transmission path by inverting the spectrum of a transmitting sound signal on a transmitting side and restoring the spectrum of a signal extracted by an adaptive echo canceler on a receiving side.

CONSTITUTION: The title device is equipped with a spectrum inverting circuit 2 to output an inverted sound signal into which the spectrum of the sound signal to be transmitted is inverted and a noise generating circuit 3 to output a noise signal in a frequency band occupied by the sound signal, and a secret call signal into which the inverted sound signal and the noise signal are added is transmitted to a transmission path 10. The secret call 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 spectrum of the signal extracted by the adaptive echo canceler 13 is restored by a spectrum restoring circuit 14 and outputted from a receiving side output terminal 15. Thus, the signal-to-noise ratio of the extracted sound signal can be improved and the secrecy on the transmission path 10 can be enhanced.