The present invention detects a failure location of network in the Internet. A pair of networks consists of an access network and an Internet service provider network, and a route consists of a source pair of networks and a destination pair of networks. The quality information values are inputted to elements in a matrix that all pairs of networks are put in same order as rows (or columns) of source pairs of networks and columns (or rows) of destination pairs of networks. If inferior quality values in the matrix line up in the row (or column), a failure is determined in a destination (or a source) pair of networks in column side (or row side) corresponding to a row (or a column). If inferior quality values line up in a slanting direction, a failure is determined in a route between the Internet service provider networks indicated to the slanting direction.