PURPOSE: To attain circuit integration by changing periodically a frequency of a talking voice signal, transmitting the result to a transmission system and applying a complementary frequency processing in synchronization with the transmission side at the reception side to simplify the circuit constitution of a privacy call communication device.

CONSTITUTION: A storage circuit 2 samples a voice signal according to a clock of a clock circuit 3 having a clock frequency f1 and stored data is read according to the clock of a clock circuit 4 having a clock frequency f2. A storage circuit 8 samples and stores a reception signal according to the clock of a clock circuit 9 in a clock frequency f3 and transmits stored data to a voice output terminal according to the clock of a clock circuit 10 in a clock frequency f4. The frequencies f1, f4 are formed to an idential constant value and the frequencies f2, f3 are changed periodically in synchronization with the transmission and reception sides.