PURPOSE: To optimize the replacement timing with an accurate judgment of the wear of a contactor by integrating an energization current value until an energization current is down to zero starting from the opening moment of the contactor sequentially at each opening to compare the results with a reference value.

CONSTITUTION: Current flowing through a contactor 1a of a switch apparatus is detected with an energization current detector 20. The opening moment of the contactor 1a is detected with an opening moment detector 30. An energization current calculator 11 calculates an energization current value until an energization current of the contactor 1a is down to zero starting from the opening moment. A device 12 for judging a degree of wear integrates an individual energization current value at each opening of the contactor 1a and when the value exceeds a reference value preset, it is outputted with a judgement that a degree of consumption by the contactor 1a reaches a fixed level. Thus, the degree of wear of the contactor 1a can be determined accurately thereby achieving optimization of a change timing of the contactor 1a.