PURPOSE: To facilitate decision of deterioration of a contact portion and the time of replacement thereof by installing such means that detects a current of the contact portion, makes a short circuit with a switch interlocking to the contact portion so that the contact portion is opened and thereupon, the switch is opened to detect k arc current.

CONSTITUTION: A current detection means 2 can detect a current when a contact portion 1 is closed, but as a means 2 is shorted because a switch 3 is closed, there are no inputs in a deterioration detecting circuit 4. On the other hand, if the contact portion 1 is deteriorated, the time of continuance of arc at the breaking time extends corresponding to the degree of advance of determination and resultantly, the breaking time extends so that a current is detected by the means 2 even after the contact portion 1 is opened. In this case, the switch 5 is opened as it interlocks to the open of the contact portion 1 and there is an input in the circuit 4 in the continuance time of arc so that deterioration of the contact portion 1 is detected. It is thereby possible to facilitate decision making of determination on deterioration of the contact portion 1 and a replacement time thereof.