A network monitoring system includes: a traffic information generating apparatus for generating traffic information. Further, the network monitoring system includes a security event collecting apparatus for collecting the traffic information generated by the traffic information generating apparatus by referring to pre-stored traffic information, grouping the collected traffic information, and then extracting service information. Furthermore, the network monitoring system includes a service abnormal condition detecting apparatus for detecting a port number of a transport layer of service information extracted from the security event collecting apparatus and the occurrence frequency of the transport layer, determining the continuity of the port number and the uniformity of the occurrence frequency, and displaying a service abnormal condition.