To prevent communication from being obstructed even when a line fault occurs in a connected line by exchange-connecting connected telephone sets belonging to the respective sides of the disconnected point in the connected line through the use of connection points to the exchanges at the respective sides of the connected line when the connected line is disconnected.

When the connected telephone set is not connected from the connected telephone set 33 to a line circuit (MPLIN) 23 executes response, it is considered that a junction line is disconnected at an A point. In this case, the instructor of a call board 30 executes connection to the junction line circuit (MPLIN) 23 where another call number is given and tries a simultaneous instruction. That is, the number of the circuit 23 is dialed and, then, a simultaneous instruction number is dialed. Thus, another connected line circuit 23 is connected and a simultaneous instruction signal is transmitted to a line side from the circuit 23.