To provide technology regarding a packet-switched network, which takes packet delays into consideration precisely and automatically.

This method is for a circuit-switched communication network which imposes strict operational requirements regarding network reliability. As the existing instrumentation technology for the circuit-switched network is not applicable to the packet-switched network, new technology is required. This invention provides the technology which takes packet delays into consideration precisely and automatically. A source unit (102) connected to one point of a tested packet-switched network (104) generates a test packet stream, and transmits the test packet stream to an analysis unit (106) connected to another point of the network. Service interruption is detected by detecting an abnormal packet, and service downtime between the last packet before interruption and the first packet after service restoration is measured.