In a network including a group of proxy servers for processing a signaling protocol for transferring signaling messages through a communication route connecting a pair of user terminals, an abnormal end of a communication session established between the pair of user terminals is detected by capturing and analyzing a signaling message received or transmitted by a proxy server. A cause section that has caused the detected abnormal end of the communication session is determined. Then, the number of abnormally ended communication sessions caused by the cause section is counted to determine an influenced range of a fault occurrence in the network on the basis of the counted number of abnormally ended communication sessions caused by each of cause sections and proxy connection information indicative of connection relations among the group of proxy servers.