A network management system comprising: a network including a plurality of packet relay apparatuses; wherein the plurality of packet relay apparatuses include first packet relay apparatuses, second packet relay apparatuses, and third packet relay apparatuses located downstream of the first packet relay apparatuses and the second packet relay apparatuses, wherein each of the third packet relay apparatuses has a first path coupled to one of the first packet relay apparatuses to send and receive traffic and a second path coupled to one of the second packet relay apparatuses and being in a blocking state, a management computer includes: a state information collection unit for acquiring state information on the first to the third packet relay apparatuses; and a power management unit for selecting a candidate packet relay apparatus to be deactivated satisfying predetermined conditions based on the state information.