When DDoS attack packets are transmitted from the attacker to the victim's server, the attack packets are detected in the edge router of the LAN accommodating the server. These packets are then destroyed, the address of the upstream routers close to the attack source are retrieved, and attack source retrieval modules are transmitted from the edge router to all the upstream routers. By executing the retrieval modules in the upstream routers, verification is performed as to whether the attack packets are passing through those upstream routers. The results are notified to the transmission source router and if the attack packets are passing through, the retrieval modules are transmitted to routers at the upper stream. When the router at the uppermost stream is reached, a protection module is executed to destroy the attack packets. When the attacks cease, the protection module deletes itself and the protection process is ended.