PURPOSE: To reduce the size of a generator by fixing an ion source vessel to the generator in such a manner that the particle flow hole thereof aligns to the particle orbit and providing a spiral or cylindrical ion forming member to the inside of the ion source vessel in such a manner that the axial center thereof aligns to the particle orbit.

CONSTITUTION: The supply of gas is stopped and the particles entering a vacuum vessel 1 are released from a particle outlet 14 through the flow hole 2b of the ion source vessel 2, the axial center part of filaments 4 and a vessel flow hole 2c when ions are not generated. The gas is supplied from a gas cylinder 11 into the vessel 2 and the filaments 4 are energized, simultaneously with which an electron drawing out power supply 8 and an ion drawing out power supply 9 are operated in the stage of generating the ions. The thermions (e) released from the filaments 4 in such a manner are accelerated by the power supply 8, by which the gas is electrolytically dissociated and the ions (i) are generated. The ions are released by the power supply 9 to the outside. The size of the generator is reduced and the safety thereof is improved if the generator is constituted in such a manner.