A system manager sets a port connected to a specific device (for example, a router device) among a plurality of ports of a switch device as a reliable port. If a packet is received in the reliable port, the switch device manages an IP address and a MAC address of the router device by a reliable port information table. When a packet is received from a port not set as the reliable port, the switch device refers to the reliable port information table. At this time, when the received packet is an address resolution packet having impersonated a router device, the switch device discards the packet without transmitting the packet, thereby preventing a cyber attack on a terminal.