A network monitoring method and system reduces a load to a monitoring network without deteriorating accuracy of detecting a malfunction. A plurality of network constituent elements that constitute a communication network are connected to an operation system through a monitoring network. A health check for detecting a malfunction is periodically performed between a group of the network constituent elements positioned closed to each other within the communication network at a short monitoring period. A health check of malfunction detection on all of the network constituent elements is performed by the operation system at a long monitoring period longer than the short monitoring period.