PURPOSE: To prevent useless line connection by stopping reception supervision of a BPDU when the state of a port transits to blocking during STP operation so as to interrupt the line.

CONSTITUTION: LANs 1, 2 are connected via a bridge and data are sent between them. When a state of ports 22b, 23b transits to a blocking state in other receiver side bridges 22, 23 not interposed in a data path of a spanning tree, reception supervision of a composition control BPDU is stopped to interrupt lines 41, 43. Upon the receipt of the BPDU representing revision of topology due to a fault of a bridge 21 or a failure of an HSD40 interposed in the HSD40 being a data path afterward, the lines 41, 43 having been interrupted are connected to attain data communication.