A network 50 is hierarchically divided into network devices 501 to 506, areas, and the like to be monitoring targets, and a monitoring device 100 monitors a divided monitoring target. A monitoring device of an N-th layer (here, N≥2) that is an upper layer with respect to a first layer monitors information of reachability between user connection points received from a monitoring device of an (N−1)-th layer that is a lower layer and network devices connecting monitoring targets of the (N−1)-th layer making up monitoring targets of the N-th layer and connection statuses between the network devices and areas that are monitoring targets. The monitoring device determines the presence/absence of reachability between user connection points belonging to different monitoring targets of the (N−1)-th layer on the basis of this reachability and the connection statuses.