A network relay device constituting a network includes a transmit/receive unit having plural ports; an FDB for storing as its registered contents at least a port identifier for identifying each of the plural ports, or an LAG port into which are grouped a specified number of the plural ports, a terminal identifier for identifying a terminal, and an FDB_ID for being associated with both the port identifier and the terminal identifier; and an FDB_ID table for storing the FDB_ID associated with the port identifier. When the registered contents of a specified port or LAG port stored in the FDB are erased, the stored FDB_ID associated with the port identifier identifying that port or LAG port is rewritten as another value.