PURPOSE: To simplify the structure by making reinforcement for a cavity part and vacuum seal performance for its penetration part unnecessary.

CONSTITUTION: A gap 24 between a pair of cylindrical electrode parts 21, 21 is blockaded by a blockading member 23 formed of non-conductive material such as ceramics so that it is kept in vacuum condition, thereby the inside of a cavity part 22 provided outside the acceleration electrode parts is partitioned from a vacuum duct 5 so as to keep the inside of the cavity at substantially the atmospheric pressure. Thus, since the blockading material is non-conductive material, high frequency electric field is produced without difficulty as matter of course and it is not necessary to make the whole of the cavity part in super high vacuum condition, thereby load of a vacuum facility can be reduced.