Spanning tree bridge is devised to minimize the service interruption time after occurrence of a fault in a network or in a component of the network by enabling faster recovery from the fault and by limiting tree reconstruction to a smallest possible area. In a bridged LAN formed of a plurality of LANs, a plurality of spanning trees are defined in one bridge such that a spanning tree is divided without dividing a LAN segment. To enable this, a plurality of spanning tree management units are provided in one bridge and are given unique bridge IDs for spanning tree management units. The arrangement is such that ports can be assigned to the spanning tree management units in a predetermined relationship. Therefore each management unit can independently perform spanning tree construction processing to divide a spanning tree in one LAN segment into a plurality ones, and a bridged LAN in which a plurality of spanning trees can exist in one LAN segment can be formed.