PURPOSE: To reduce the amount of arithmetic operation on both transmission and reception sides without degrading the ciphering secrecy and the cipher intensity by performing encoding only important bits that characterize voice information or the like in encoding an information source, and then performing error detection or the like and the transmission.

CONSTITUTION: Appropriate ciphering is performed only to the accoustically characteristic most important bits, CRC codes are calculated and added to them and error correction coding and inter-slot interleave are performed. On the reception side, decoding processings corresponding to the operations are advanced in the opposite order of the encoding and voice data are decoded. Thus, integration is performed while utilizing the features of required low rate encoder, ciphering device and error detection and error correction devices in digital communication and ciphers are made safe and easily realizable. That is, since the ciphering is performed only to the characteristic important bits, making into a hardware is facilitated.