A network security monitoring apparatus and a network security monitoring system manages “permitted” or “not permitted” communication between nodes based on an access policy. A network security monitoring system includes nodes 31,32,33, application server 20, router 40, and network security monitoring apparatus 10 deployed in the network. The network security monitoring apparatus 10 judges whether the nodes are permitted to communicate with other nodes in the network or not based on the access policy, and repeatedly transmits data to block the communication between nodes judged as “not permitted” at fixed time intervals until the access policy is changed from “not permitted” to “permitted”. This invention enables to block communication between nodes defined as “not permitted” for communicating with other nodes in the access policy, and to allow communication between nodes defined as “permitted” for communicating with other nodes in the access policy.