PURPOSE: To discriminate whether an alarm is original for a device or abnormal, by connecting a resistor in parallel to an alarm contact in the middle of a cable, composing a potential divider circuit of the resistor and a resistor on the device side and comparing the output of this potential divider circuit with a prescribed reference voltage.

CONSTITUTION: Since an alarm contact 11 is closed at normal time, the potential divider circuit is not constituted but a first potential dividing voltage V1 is made almost equal to an impressing voltage E. When an alarm is generated, the contact 11 is opened and the first potential divider circuit is formed by resistors R and R1. Then, the potential dividing output V1 is expressed by a formula. In respect to this state, when abnormality is generated at the cable, the voltage V1 is made almost equal to a ground potential 0. At such a time, when the alarm is generated, in respect to second and third potential dividing voltages V2 and V3, because of V2>V1>V3, only a comparator 22a is inverted and the output is made '1'. An inverted signal is passed through an AND circuit 22c and becomes a signal '1' and an alarm signal is outputted. Next, when the cable is disconnected, the potential of the V1 of 0 and the output of a circuit 22c is '0'. Then, the output of an AND circuit 22b is '1' and the other abnormality is shown.