PURPOSE: To improve the deterioration of quality of a privacy signal by controlling the change in a privacy call converting parameter by taking notice of the privacy system where the privacy call converting parameter is changed continuously.

CONSTITUTION: The privacy call conversion 4 is performed by using the output of a privacy call converting parameter signal generating circuit 3 inputting a digital input signal 2 to an analog input signal 1 as a parameter. The digital signal is ciphered 5 and synthesized 6 together with a privacy call analog signal and transmitted 7 to an output terminal 8. The synthesis signal received by a reception circuit 27 is separated 26. The ciphered digital signal is decoded by a ciphering decoding circuit 25 and the transmitted digital signal is obtained at an output terminal 22. A part of the digital signal is inputted to a privacy call inverting parameter signal generating circuit 23, the privacy call analog signal is decoded by the privacy call inverting circuit 24 and a transmission analog signal is obtained at an output terminal 21.