PURPOSE: To realize high quality prediction of fault by detecting the discharge phenomenon, which takes place prior to electric fault, in the form of a voltage having high S/N which can be measured or compared easily.

CONSTITUTION: Upon occurrence of a discharge fault at some point in a circuit, it is detected by detecting sections A located at the joints 14 of a plurality of ground lines 12 of the metal housing 10 for electric facility being applied with a voltage from a normal power supply 7 and a ground electrode 15 within the coverage in the form of a voltage having high S/N which can be measured with the same frequency and can be compared easily. The detected output is fed to an electric circuit 4 where the discharging point is selected and the state thereof is grasped. It is then compared with an allowable value of the electric facility and the results are displayed on a control board 5 and an alarm is delivered, as required. The results are also notifies automatically to a normal supervisory station 6 performing central remote control. This method allows maintenance of electric facility based on a high quality fault prediction.