PURPOSE: To obtain a withstand voltage performance of a vacuum circuit breaker by providing a measuring means for detecting a preceding discharge thrown at a pole closing operation and measuring the distance between poles.

CONSTITUTION: A start for throwing operation of the circuit breaker 3 is detected by a rotary encoder 4. An operation angle of operating shaft is obtained from the pulse output generated from the rotary encoder 4. The beginning of a preceding discharge current made to flow is detected by a signal ILED from an optical fiber 14 and the distance ls between poles is calculated by the rotary encoder 4. The output E of a differential amplifier 10 at that time becomes an impressing voltage Es between poles. The withstand voltage between poles is obtained by a data processing circuit 15 based on the impressed voltage Es between poles at the time point when the preceding discharge current is begun to flow and the distance ls between poles. This withstand voltage between poles is compared with the reference withstand voltage characteristic between poles which is being stored inside beforehand, and also the change of this compared value is trend-processed, then a deterioration of the withstand voltage between poles is diagnosed to issue an alarm.