A transmitting side relay device comprises additional information generation means for generating additional information about the characteristics of a data stream, packet generation means for generating a packet by attaching additional information to each transfer unit and transmitting means for transmitting the packet to the third network according to prescribed procedures. A receiving side relay device comprises separation means for breaking down the received packet into additional information and a transfer unit, analysis means for analyzing the additional information, reproduction means for locating information included in a corresponding transfer unit in an appropriate position and reproducing the data stream and output means for outputting the data stream reproduced by the reproduction means to the second network.