PURPOSE: To surely prevent the risk of a microwave leak by tieredly forming the transitive part from the tubulous part to the flange part of an ion source head, tieredly forming the inner side of the flange part of a vacuum vessel in closely engaging the tiered part, and controlling the oscillation output of a microwave oscillator with the pressure within the vacuum vessel detected.

CONSTITUTION: The transitive part from the tubulous part to the flange part 11a of an ion source head 11 is tieredly formed, also the inner side of the flange part 55a of a vacuum vessel 55 is tieredly formed in closely engaging the tiered part, and the oscillation output of a microwave oscillator is made to be controlled by the output signal from the means provided for detecting the pressure within the vessel 55. At the point of time of the start of the flange clamping, both flanges 11a and 55a become a closely engaged state already, and a microwave leak to outside is unocurring even probable insufficient or uneven clamping. Moreover when a given pressure within the vessel is unattainable, the microwave oscillator is controlled. This prevents the risk of a microwave leak even it assembly is made in an insufficient engaging state of the tiered part.