PURPOSE: To increase the dielectric strength between a pin-like chip and an opposed electrode, and make an ion gun compact by providing an insulator block, which supports the electrode and electrically insulates the electrodes from the outside of the ion gun, with grooves of specified dimensions so that the electrodes are surrounded by the grooves.

CONSTITUTION: An electrode 3 is provided with a pin-like chip 1 used for ion generation. Grooves 8 are provided on, for example, the upper and the lower surfaces of an insulator block 4 which supports the electrode 3 and electrically insulates the electrode 3 from the outside of an ion gun. When the outer diameter and the thickness of the insulator block 4 are supporsed to be D and T respectively, the width (W) and the depth (l) of the grooves 8 are restricted within the range D/50≤W≤D/3 and T/50≤l≤T/2. By thus providing at least one groove 8, which has dimensions within the above ranges, around the electrodes 3, the voltage resistance of the ion gun can be widely increased.