An IPS examines a communication packet exchanged between an internal communication network and an external communication network, and detects a particular packet that satisfies a particular condition. The IPS creates a detection request message which includes a request to detect the source of the particular packet and the destination of which is set to the address (virtual address) of the virtual server, and transmits the created message to an SLB, upon detection of the particular packet transmitted from the internal communication network. An RIP detecting device extracts a real address indicated as the destination in the detection request message upon receipt of the detection request message the destination of which is rewritten by the SLB from the virtual address to the real address of the server, which is the source of the particular packet.