PURPOSE: To detect abnormality of a partial discharge measuring apparatus without missing the partial discharging data even in a case where the noise level is low.

CONSTITUTION: An input is compared with a threshold value Vth in a comparator 2. A threshold-value setting circuit 1 determines the threshold value Vth at any time. The comparator 2 generates a reading signal for an input Vj exceeding the threshold value Vth. The threshold value Vth is changed in correspondence with the magnitude and the frequency of the input signal so that the number of the input signal larger than the threshold value Vth in the unit time is always contained in the certain range. When the noise level is low, the threshold value Vth becomes low, and the value is measured through the th low noise level. Therefore, it is found that the measuring instrument is defective. When the noise is large, the threshold value Vth is increased, and the input of the noise is suppressed. Even in this case the signal of the partial discharge can be measured without missing.