A system and method for exactly estimating a volume of traffic is provided when packets are transferred within the network. The system and method receive data packets from clients through a plurality of router-for-clients, and creating a flow information packet for estimating information about a flow, transfer the data packets and the created flow information packet to the destination router-for-client via a router-for-relay, calculate information about the traffic of a flow at the destination router-for-client through the flow information packets transferred from the router-for-clients, and estimate a volume of the traffic. Thus, the system and method can estimate the volume of the traffic on the network with respect to the aggregated flow defined by the router pair connected to the terminal or the external network when the traffic is actually introduced into the network, instead of the flows of each terminal and each destination.