Flow information of excessive traffic detected by a traffic monitor of a packet forwarding apparatus is entirely delivered to a manager server. By combining input/output port information of the flow and topology information, the manager server calculates from where the flow is inputted and to where the flow is outputted. By further combining results of judgment of traffic contents, the manager server controls the traffic at an optimal position according to the traffic contents. The problems of inefficiency can be avoided by managing the flow information by the manager server in a concentrated way. Adding the flow information to the network topology drawing for each flow, it is possible for the network administrator to more easily understand the statuses of flows on the network.