PURPOSE: To ensure the quick information of a fault for an unmanned operation of a data communication system, etc., by providing a function to dial a specified telephone number.

CONSTITUTION: In the normal network control mode, a state monitoring part 4 monitors the transmission and reception of the signal 8 and the signal 9 to a modulating/demodulating device, etc. and a data terminal device, etc. respectively. The part 4 informs the occurrence of a fault to a control part 2 when the fact that the transmission/reception of the above-mentioned signals is carried out by a procedure other than the normal one is detected. Thus the part 2 discontinues the control of the network, dials automatically the telephone number of a telephone number setting part 3, transmits the signal decided at a signal transmitting part 5 after receiving an answer from the remote subscriber side and informs a fault of the data communication system.