The monitoring method includes: detecting a cut link among a plurality of links forming a link aggregation; counting lost packets related to remaining links in the plurality of links excluding the cut link to obtain a first lost packet count for each first address included in packets passed through the remaining links; sorting the first lost packet counts into first segments corresponding to the remaining links; determining degradation in quality for each first segment on the basis of the first lost packet counts sorted into the first segments; determining whether the cut link is in failure on the basis of the determined degradation in quality for each first segment; and outputting a determination result of whether the cut link is in failure.