PURPOSE: To prevent interception by the 3rd party by superimposing a pseudo random signal and a pilot signal on a voice signal, transmitting the result and subtracting the pseudo random signal formed from the pilot signal from the reception signal at the reception side to obtain the original sound signal.

CONSTITUTION: The voice signal Sa from a transmitter 11 is inputted to an adder circuit 14. The pilot signal Sp is formed by a pilot signal forming circuit 21 and fed to the circuit 14. Then this signal Sp is fed to a PLL 22, a signal Sn not including unnecessary high-order high frequency component is formed through a pseudo random signal forming circuit 23 and a low pass filter 24 and fed to the circuit 14. Thus, a summing signal Ss among the voice signal Sa, the pilot signal Sp and a noise signal (n) is extracted from the adder circuit 14. The pseudo random signal Sr is generated from the pilot signal Sp at the reception side, it is subtracted from the reception signal and the original voice signal is obtained.