PURPOSE: To easily discriminate address overlap with which plural terminals provided with the same address are made existent.

CONSTITUTION: When a frame from a terminal C is received at bridge devices 10-1-10-3, for example, address learning tables 12-1-12-3 are retrieved with an address (b) of that terminal C, when address information containing the address (b) is registered, it is investigated whether a branch line LAN address in the same address information is matched with the address of a branch line LAN interface 11-3 to which the terminal C is connected through a network n3 or not, when those addresses are not matched, the address information concerning the transmission source terminal C is registered on overlap address tables 13-1-13-3 by judging the existence of plural terminals provided with the address (b) and even when the address information concerning a terminal B provided with the address (b) is registered on the tables 12-1-12-3, that address information is not reloaded.