A packet relay device receives an input packet via a first communication port out of a plurality of communication ports of the packet relay device and transmits the input packet via a second communication port out of the plurality of communication ports. The packet relay device includes a learning table storage, a header adder, a discard determiner, and a packet reducer. The learning table storage stores a source address contained in the input packet, in association with an input port identifier capable of identifying the first communication port. The header adder adds an internal transfer header containing the input port identifier to the input packet to generate an internal transfer packet. The discard determiner determines whether to discard the internal transfer packet. The packet reducer reduces the internal transfer packet to preserve at least the source address and the input port identifier upon determining to discard the internal transfer packet.