PURPOSE: To attain sure fault detection and alarm corresponding to the versatility of telephone sets by providing a circuit detecting a fault of a telephone line and a terminal equipment and alarming the result of detection to each terminal equipment.

CONSTITUTION: If a fault (for example, open wire or permanent call) takes place in a telephone line 1 or a terminal equipment 2, the voltage or current of the telephone line 1 changes normally. The change is detected by a line monitor circuit 11 to detect the fault. In detecting the fault, an alarm circuit 12 is operated and an alarm (such as alarm sound or display) is given to the terminal equipment. Then an equipment detecting the fault and alarming it is provided to the terminal equipment side to allow the titled controller to cope with the versatility and many kinds of the equipments more than in comparison with the case that a central exchange implements all kinds of fault detection easily. Moreover, the fault is detected, then sure alarming is given to the terminal equipments independently of the location or kinds of the fault.