PURPOSE: To execute reliable privacy communication by converting quantized and coded data based on a conversion table and inversely converting the conversion based on the conversion table so as to restore an original signal.

CONSTITUTION: On a transmission side, A/D converted 1 PCM data is ciphered by a code converter 2 based on the conversion table and the ciphered PCM data is coded in an encoder 3 to be transmitted after four-phase DPSK modulation is executed. Now, the conversion table of the converter 2 is regularly updated. The signal is received on a reception side and the four-phase DPSK modulated signal is demodulated in a demodulator 5 to obtain an original bit stream signal. The bit stream is transmitted to a decoder 7 with contract channel information and decoded, so that only contract channel data is decoded and fetched.