PURPOSE: To remove the reduction of transmission efficiency by adding synchronous information to adding information in the adding information of transmission data.

CONSTITUTION: On the transmitting side, adding information from a subdata output circuit 7 is applied to main data divided into blocks by a main data output circuit 6 and the added data are inputted in parallel to two CRCC generators 9, 10 having different generator polynomials to generate CRCC. The output of the generator 9 when the adding information of the subdata output circuit 7 is located on the leading position of a subdata block or the output of the generator in case of the location on the way of the block is selected and transmitted through an adder 12. In the receiving side, CRCC operators 13, 14 corresponding to two kinds of generator polynomials are prepared, errors of the transmission system is detected by an NOR gate 16 and the synchornous signal of the adding information is detected by the output of an operating circuit 14.