PURPOSE: To always monitor the consumed quantity of the contact of a switching device without cutting off the current of a main circuit by accumulatively adding the data of an effective value of current at the time of interrupting the current of a current switching device.

CONSTITUTION: Current is interrupted by opening a contact within a current switching device, and the high-frequency component and the commercial frequency component involved with the generated arc-current are detected by a high-pass filter circuit 3 and a low-pass filter circuit 5 through a converter 2, and the level of the high-frequency component at the time of interrupting current is determined by a level determination circuit 4, and when it exceeds a certain value, a pulse signal is outputted to a waveform memory circuit 6. The commercial frequency component is detected by the low-pass filter circuit 5 and outputted to the waveform memory circuit 6. A waveform stored in the waveform memory circuit 6 is operated by an effective value operation circuit 7, and the effective value of current at the time of interrupting current is operated, and the effective value of current is accumulatively added by a data accumulation circuit 8 at the time of interrupting current, and when the accumulatively added data becomes not less than a certain value, an alarm is given out, so that the consumptions of a contact may be always monitored with accuracy without a main current is interrupted.