A plurality of result signals are driven low when abnormal condition is detected and driven high when no abnormal condition is detected. Presence and absence of the abnormal conditions of different types are indicated in a predetermined sequence by high and low signal levels of the result signals. The voltage detecting circuits simultaneously output the result signals on corresponding branching lines connected to a main line of a communication line. An OR gate outputs a result signal on which the result signals output by the voltage detecting circuits are superimposed. When at least one of the result signals of the voltage detecting circuits is at a level indicative of the presence of the abnormal condition, the OR gate outputs on the main line the result signal having the level indicative of the abnormal condition.