PURPOSE: To improve the secrecy of talking by changing a clock frequency when a voice signal is inputted to a delay circuit to make it different from a clock frequency at an output after the delay.

CONSTITUTION: The clock frequency to the delay circuit 3 is changed by changing the frequency dividing number to make the frequency of a clock pulse 21 when a voice signal is inputted to the delay circuit 3 from the clock frequency at the output after the delay different. The frequency of an output voice signal is changed to the original frequency in this way to scramble the voice. On the other hand, the clock frequency to a delay circuit 14 at the reception side is changed in synchronization with the change in the clock frequency to the delay circuit 3 at the transmission side in the circuit of the same constitution as the transmission side. Thus, restoration is performed by the conversion again so that the frequency of the received scrambled voice is restored to the original frequency in this way.