PURPOSE: To prevent omission of detection signals by connecting a protection resistance in series to an output terminal of a high voltage generator and a bypass capacitor in parallel to the power source section to lower the impedance of the power source section.

CONSTITUTION: A bypass capacitor 13 is connected in parallel to output terminals A and B of the power source section in which a protection resistance 3 is connected in series to a high voltage output terminal of a high voltage generator 2. The electrostatic capacitance of a bypass capacitor 13 is selected to be an equal value to the electrostatic capacitance of a connection capacitor 4. With such an arrangement, the impedance of the power source section can be lowered so that the detection sensitivity of the detection signal v3 to be detected with a detector 9 can be elevated to the equal level to the detection signal v1 to be detected with a detector 7 with respect to a partial discharge generated in a sample 1. Thus, the impedance of the power source can be lowered.