A network traffic analyzing device, method and system are provided that reliably analyze network traffic with high precision, and reliably analyze alert generation cause. The network traffic analyzing device accurately analyzes traffic of a communications network. The traffic analysis device includes a real time monitoring unit configured to collect information regarding communication data between a primary network and an access network from a traffic collecting device in real time; an alert managing/notifying unit that generates an alert regarding traffic between the primary network and the access network based on the information collected in real time by the traffic collecting device; and an alert generation cause analyzing unit that analyzes a cause of the alert generated by the alert managing/notifying unit based on information regarding at least one of normal data and abnormal data transmitted and received between the primary network and the access network prior to generation of the alert by the alert managing/notifying unit.