To prevent the wasting of wireless resources when a link speed significantly drops in one of paths in a redundant network.

A receiving device includes: a receiver which receives packets from a transmitting device over a plurality of paths; and a transmitter which generates reception-acknowledgement information indicating that the receiver received a packet, and which transmits the reception-acknowledgement information over a path which is, among the paths, other than the path over which the packet was received. The relay device includes: a receiver which receives packets; a transmitter which sequentially transmits the packets received by the receiver; a reception-acknowledgement information receiver which receives reception-acknowledgement information; and a packet discard controller which discards, based on the reception-acknowledgement information, a packet which is not yet transmitted by the transmitter.