PURPOSE: To enable highly accurate measurement by finding the drop voltage by interelectrode impedance and the induced voltage by electromagnetic induction from the current at the time of an arc discharge, and by correcting the detected voltage.

CONSTITUTION: The voltage is applied from a generator 2 to a breaker 4 in the state in which the breaker 1 is shut, and the output V1 of an interelectrode voltage detecting circuit 3 and the output V2 of a variable impedance circuit 6 are measured by an oscilloscope. The output V1 corresponds to the sum of the drop voltage VZ by interelectrode impedance Z and the induced voltage VN by electromagnetic induction in this time. If the impedance ZV of the circuit 6 is controlled so that the output becomes V2=V1, the output becomes V2=VZ+ VN. An arc discharge is generated between electrodes of the breaker 1 after this, the measurement is made and the output V0 of a subtracter 8 is made the measured value of an arc voltage Va. The correctly measured value of the voltage Va can be found because the output becomes V0=Va=V1-V2=V1-(VZ +VN) in this time.