PURPOSE: To separate a pulse current attributed to a partial discharge from an electric noise and make possible measurement at positive and negative polarities by combining diodes which run positive-polarity current and negative-polarity current.

CONSTITUTION: If a partial discharge occurred in the equivalent electrostatic capacity of a test sample, an electric current is, is' runs, causing a negative-polarity current i2 to run through a diode 8 and a positive-polarity current i1 through a diode 9. Then these currents flow into an output impedance Zp. In this case, the positive- and negative-polarity currents i3, i4 due to the current is' run through D15, 12 and 13, 14 respectively. In addition, the currents iN, iN' run according to polarity on account of an electric noise generated by an impedance Zp on the power supply side, but to not run through D8, 9. Consequently, in case a reduced oscillation waveform of the current is runs, the terminal voltage of Zp is equal to the waveform of the current is. The currents is', iN, iN' do not run through Zd, so that a pulse current attributed to a partial discharge is detected at Zd of a detector 16 and a noise voltage generated at Zp is not detected.