A control PVC is set up on a line connecting between exchange units, and periodic communication is performed between the exchange units using the control PVC. The periodic communication is monitored at each exchange unit for detection of a line failure and, at each exchange unit, a connection on the line is automatically switched over to a reserve connection in the event of a line failure. For manual switchover of a connection set up between the exchange units, a manual switchover request is issued from a maintenance console connected to one of the exchange units and, in response to the request, switchover request information is sent to the other exchange unit. The exchange unit that received the switchover request information switches the connection over to a reserve connection, and returns a switchover completion notification. The exchange unit that received the switchover completion notification switches the connection over to a reserve connection.