The communication availability information between a plurality of transmitters and a plurality of destinations before the communication state between a transmitter and a destinations in a communication network changes from available to unavailable is compared, layer by layer from an upper layer toward a lower layer, with the communication availability information after change, and destinations in paths where the communication state has changed from available to unavailable are grouped. Thus, the communication network is divided into a plurality of affected ranges and an interface of a device located at the border between the affected ranges is deduced as a fault occurrence point.