PURPOSE: To easily detect a partial discharge during the operation of an apparatus by a method wherein AE waves at a time when a signal on one side is at a threshold value corresponding to the electric charge amount of a discharge or higher are discriminated from those at a time when a signal on the other side contains a frequency component at a specific wavelength.

CONSTITUTION: An output from an AE sensor 21 is divided into two identical signals through a buffer amplifier 22. A signal A is amplified by a preamplifier 25 through a high-pass filter 23 at, e.g. 50kHz, it is amplified by a main amplifier 26, and it is connected to a channel on one side of an AE analyzer 27. A signal B is amplified by a preamplifler 25 through a bandpass filter 24 at, e.g. 300 to 600kHz, it is amplified by a main amplifier 26, and it is connected to a channel on the other side of the A9 analyzer 27. When the signal A which has reached the AE analyzer 27 is set in advance at a threshold value corresponding to the electric charge amount of a discharge to be detected, it is detected as a partial discharge when a frequency of, e.g., 300 to 600kHz is contained in the signal B.