An apparatus relays packets transferred over a network and discards an attack packet detected among the packets. The apparatus includes: an inspection-packet outputting unit that outputs, when detecting the attack packet, an inspection packet in which a transmission-source address contained in the attack packet is set as a destination address and a destination address contained in the attack packet is set as a transmission-source address; a filter table storing unit that stores, when acquiring a response packet for the inspection packet, a transmission-source address, a destination address, and identification information of an interface, which has received the response packet, that are contained in the response packet, in a filter table in an associated manner; and a transfer control unit that determines whether to transfer a packet as a transfer object based on the filter table.