PURPOSE: To prevent failure of an electron bombardment circuit when degassing is carried out in a device having an ion source with an electron bombardment system degassing function.

CONSTITUTION: An electric current loop 11 to flow a discharge current Is generated by discharge between an anode 3 and an earthing part when an electron is bombarded, is formed separately from an electric current loop of a thermonic current Ie, and a voltage detector 12 is arranged in the electric current loop 11 of the discharge current Is so as to detect only the discharge current Is flowing through this electric current loop 11 as a voltage value. A control circuit 13 is arranged to generated an output signal to operate a protection network 15 when the voltage value exceeds a reference voltage value Vs by comparing the voltage value detected from this voltage detector 12 with the reference voltage value Vs.