A device and method for designing a reliable communication network using disjoint paths with no shared links or nodes. An adjacency matrix is copied to a temporary adjacency matrix, and when it has been confirmed by matrix calculation that there is a path or paths between the origin and destination nodes, a path is obtained backwards and links in the path are deleted. The search for a path is repeated using the resulting temporary adjacency matrix. In this way, the actual number of disjoint paths is never overestimated. Rapid calculation is possible by means of a super-computer.