In a network system according to the hierarchical routing method, in which at least one main LAN and a plurality of sub-LANs are connected hierarchically through bridges, bridge IDs having the smallest values in the network are given to a plurality of bridges (first bridges) connected to the main LAN. Bridge IDs unique to each other having values greater than the bridge IDs of the first bridges are given to a plurality of bridges (second bridges) connecting sub-LANs. Each of the first bridges transmits periodically a control message (Hello message) for making the network in a tree structure by using some of the second bridges as backup bridges to the side of the sub-LANs. When a change takes place in the topology of the network, the reconstruction of the tree structure is effected in a localized part of the network including a position of hindrance, the root of the network being constituted by a particular first bridge.