An attack defending system allows effective defense against attacks from external networks even when a communication system uses a communication path encryption technique such as SSL. A firewall device and a decoy device are provided. The firewall device refers to the header of an input IP packet and, when it is determined that the input IP packet is suspicious, it is guided into the decoy device. The decoy device monitors a process providing a service to detect the presence or absence of attacks. When an attack has been detected, an alert including the attack-source IP address is sent to the firewall device so as to reject subsequent packets from attack source.