To prevent throughput from being reduced, by saving unnecessary polling from a master agent to a slave agent that is not installed yet on a network.

The present invention relates to a network monitoring system wherein a manager monitors a plurality of agents and network line faults. For the plurality of agents, a subnet group is defined which is constituted of a plurality of slave agents and a master agent that performs polling on the plurality of slave agents, the manager includes functions for presetting an address of each of slave agents to stop polling and the number of the slave agents to stop polling and transmitting setting information containing set contents to the master agent, and the master agent includes a function for stopping polling to the slave agents under management based on the setting information received from the manager.