Discharge factors existing on a surface of an electrode or an insulator forming an electron gun are removed efficiently and effectively, thus simply and easily enhancing the withstand voltage property of the electron gun. A conditioning processing device of an electron gun is provided with a voltage supply section, a voltage adjusting section for adjusting the output voltage of the voltage supply section, and a current detection section for detecting a leakage current flowing between the electrodes of the electron gun. Further, there are attached a vacuum exhaust section for adjusting the inside of the electron gun in a reduced pressure condition and a pressure detection section. Further, it is arranged that a personal computer (PC), for example, performs data processing based on the leakage current detected by the current detection section or comparison with a reference value thereof to control the voltage, which is applied between the electrodes from the voltage supply section via a connection section, via the voltage adjustment section.