A terminal device 1 collects RTP information related to a call and transmits the RTP information to a CSCF 3. The CSCF 3 collects, from a plurality of SIP packets related to the call, the address of an edge router 2 that has handled the plurality of SIP packets, the address of an SBC 4 that has handled the plurality of SIP packets, and the domain name of an operator's network 5 that has handled the plurality of SIP packets, and stores in a call information list, for each call, the collected information in association with the RTP information transmitted from the terminal device 1. Then, the CSCF 3 identifies a plurality of problem-occurring calls using the RTP information, and determines a faulty portion based on a bias in the quantitative trend of the count of each of the edge router 2 addresses, the count of each of the SBC 4 addresses, and the count of each of the operator's network 5 domains.