PURPOSE: To permit to continuously operate an ion source in such a condition that it has the rare occurrence of electric discharge as well as stability in operation, by heating the surface of an electrode to a high temperature for suppressing the deposit of foreign matter on the surface of the electrode.

CONSTITUTION: A heater plate 21a and an insulating sheet which insulates the plate 21a electrically from an electrode are respectively buried within an intermediate electrode in an ion source, and then the surface temperature of the intermediate electrode can be raised to a required value by adjusting the level of an electric current flowing through the plate 21a. This makes it possible to prevent the deposit of foreign matter on the surface of the electrode, that is responsible for the occurrence of electric discharge between electrodes which accelerate ion beams in the ion source. Accordingly, the ion source can be continuously operated in such a condition that it has the rare occurrence of electric discharge as well as stability in operation.