A discharging part of an X-ray generator using a one-side earthed X-ray tube is earthed is identified on the basis of the tube voltage detected value and the tube current detected value. For the identification, the X-ray generator comprises a device comprising tube voltage decrease slope calculating means (S4), tube current increase calculating means (S4), first judging means (S5) for judging whether or not the slope of the tube voltage decrease exceeds its acceptable value, second judging means (S6) for judging whether or not the increase of the tube current exceeds its acceptable value, and discharge portion identifying means (S7, S8) for identifying the discharging part which is in the X-ray tube or a high-voltage generating unit on the basis of the results of the judgments made by the first and second judging means. The identified discharging part is displayed on display means (S9).