In a packet transfer system, a transmission-side relay device extracts and integrates data included in a packet transmitted from a transmission device to obtain a packet data stream, extracts data from the packet data stream to obtain a data block, encodes the data block with an error correction code to generate redundant data as a redundant block, generates an error correction data stream obtained by integrating the data block and the redundant block, and packetizes the error correction data stream and transmits the packetized error correction data stream. A reception-side relay device restores missing data using redundant data included in the received data in a case in which there is missing data. The system enables packet transmission and reception to be efficiently performed even in a high loss rate environment.