PURPOSE: To detect the recovery of a fault of a terminal equipment line during communication through a backup line by making packet protocol processing units in duplicate.
CONSTITUTION: A packet communication path for data terminal equipments 11, 18 respectively connecting to packet exchange systems 10, 19 is set as a path from a terminal equipment accommodation circuit 21 to a time division switch 24 and a packet protocol processing unit 23 by a central controller 33 to form a terminal equipment line connection 25. The communication path when the terminal equipment line 12 or the packet protocol processing unit 23 is faulty is set as a path from a backup line 16 formed by a line exchange network 13 to a trunk 27, the time division switch 24 and the packet protocol processing unit 28 by the central controller 33 to form a backup line connection 29. When the terminal equipment line 12 is faulty especially, the packet protocol processing unit 23 monitors the fault state to detect fault recovery.