[OBJECT] To provide a quality-degraded portion estimating apparatus capable of accurately and rapidly estimating a portion of a communication network system where communication quality is degraded according to flow quality information.[CONSTITUTION] A quality-degraded portion estimating apparatus includes: a flow quality collection unit (11); a route information collection unit (12); a flow quality/route link table managing unit (13); a non-degraded link removing unit (15) for deciding a quality-degraded flow and a quality-non-degraded flow according to a degrading threshold value and a non-degrading threshold value and eliminating a non-degraded link having the number of quality-non-degrading flows which are passed is equal to or more than a non-degrading flow number threshold value; and a quality-degraded portion estimating unit (17) for outputting as a quality-degraded portion, a subset containing a minimum number of elements among the subsets of a link set obtained by eliminating a non-degraded link from the link set through which an arbitrary flow set which has caused quality degradation passes.