This invention is to minimize influence to other network by preventing unauthorized accesses such as DDoS attacks and probing by worms. When the blocking apparatuses 10a and 10b detect outbound packets by the DDoS attacks or by the probing by the worm, they carry out the Egress filtering for such outbound packets to prevent the packets relating to the unauthorized access from being sent to the backbone network 1000. Moreover, because a notice to the effect that the unauthorized access is detected is sent to other blocking apparatuses 10c and 10d via the management apparatus 16, for example, the blocking apparatuses 10c and 10d precautionarily carry out the Ingress filtering to prevent the packets relating to the unauthorized access from being sent to the network C and D.