In accordance with an instruction from a network monitoring device (4), a transmission terminal (2T) serving as a source of video information transmits a certain amount of monitoring test packets differentiated from the usual multicast frames. Pass status information (for example, MIB information) generated by routers (3) receiving and passing through the continuous test packets is collected at the network monitoring device (4). The device (4) uses the collected pass status information and topology information which the device (4) holds by itself so as to judge the direction of transmission of the test packets at the interface of each router (3) and specifies the multicast tree based on the results of judgment.