PURPOSE: To contain a specific signal in a frame of a parity frame system by using a bit allocated to a specific line of a multi-frame as a control bit, and containing plural signals in the multi-frame.

CONSTITUTION: Outputs of DS2 stuff circuits 11-14 of a transmitting side are multiplexed by a multiplexing circuit 14 together with a DS1 input of other channel, as second, sixth and seventh channel signals, respectively, and a necessary signals is inserted therein by a C bit parity frame inserting circuit 15 and outputted as a DS3 signal. In this case, the C bit partity frame inserting circuit 15 inserts information related to a stuff processing in the DS3 stuff circuits 11-13 into a C1 bit of a multi-frame. In a receiving side, a necessary signal contained in the received DS3 signal is extracted, and also, demultiplexed. Outputs of second, sixth and seventh channels of a demultiplexing circuit are subjected to destuff processing, based on control information extracted from the C1 bit.