To enhance the secrecy of communication data by applying bit shift processing to shift a data string with an error correction code added thereto to a tail end of the data string by a prescribed amount from the head of the data string and sending the resulting data string with shift amount data.

In the cable telephone system utilizing a network of a 2-way broadcast system, transmission data are fed to a transmission data generating circuit 54, in which the data are divided into data bit strings and an error correction code addition circuit 55 adds an FEC error correction code to the tail end of each data string. Based on shift amount notice data outputted from the transmission data correction circuit 54, the data string of one unit consisting of an information bit string + FEC is divided to conduct the rearrangement processing to shift bits ranging from the head to a bit designated by the shift amount notice data, to the tail end of he FEC. Then the result is outputted to a transmission line via an output circuit 57 with the shift amount notice data. A receiver side conducts reverse processing to the processing above to make demodulation. Thus, the secrecy of communication data is enhanced with a simple configuration.