Failure part is detected using calculated structure information. In one embodiment, the failure part is detected by comparing path node positions, numbers of node positions, branch lengths, and branch positions. That is, it is detected that although nodes a2 and a3 exist in structure information N in phase N, corresponding nodes do not exist in the preceding phase N-1 or the subsequent phase N+1. Moreover, it is detected that a node a1 existing in the preceding phase N-1 and a node a5 existing in the subsequent phase N+1 do not exist in a corresponding part a4 in the structure information N in the phase N.