Handling a packet according to a gateway rule in a network gateway device ensures security. Centrally controlling the gateway rule in an administration center eliminates the need for individual user's consciousness of operation such as a firewall, which makes a network gateway system easy to use. Also, under certain conditions, the packet is handled according to a gateway rule in the network gateway device to reduce a network load. The administration center that holds and creates a gateway rule is provided. The network gateway device holds the gateway rule. When receiving a packet, if a gateway rule corresponding to the packet exists, the received packet is handled according to the gateway rule. If no gateway rule corresponding to the packet exists, the packet is transferred to the administration center, and then the corresponding gateway rule is received from the administration center so that update is performed.