To provide a congestion monitoring method and system in which a spot where a congestion occurs and spots possible to incur congestions can be specified.

A test packet stream is generated for forming a traffic pattern in a time-sequentially predetermined cycle of a quantity of generation per unit time. The generated test packet stream is sent to a packet network. A traffic volume of a packet to pass is detected as time-sequential data by a router or a bridge in the network. The time-sequential data of the detected traffic volumes are collected, and a frequency analyzing processing is applied to the collected time-sequential data to find a spectrum strength. The corresponding relation of the spectrum strength resulting from frequency analyzing processing and a spectrum strength of the test packet stream in the case of sending to the network is decided and when the corresponding relation is not recognized between the spectrum strength of the traffic volume detected as time-sequential data and the spectrum strength of the test packet stream in the case of sending to the network, the occurrence of a congestion is decided.