PURPOSE: To restore a system to a virtual line and a status before resetting by means of restarting a network program even if terminals are reset by providing a network address storage table, a virtual line information table and a status information table on a line concentrator.

CONSTITUTION: The network address storage table 41 in RAM 11 registering the network addresses of the terminals 31-34, virtual line information and status information, the virtual line information table 42 and the status information table 43 are provided on the line concentrator 35. At the time of resetting the terminals 31-34 and of removing the network program, CPU 13 registers virtual line information and status information of the terminals 31-34 in RAM 11 and transmission and reception can be executed with an opposite party with whom the terminals 31-34 establish a virtual line until the network program is restarted. Thus, the system can be restored to the virtual line and the status before resetting by restarting the network program even if the terminals are reset.