PURPOSE: To extend the range for matching the impedance between a microwave oscillator and a microwave ion source, and precisely conduct this matching in a short time.

CONSTITUTION: A waveguide 8 for connecting a microwave oscillator 6 to a microwave ion source 2 is provided with a reflected power detector 10 for detecting the reflected power of a microwave from the microwave ion source 2 side and three stubs 21-23 which can be mutually independently put into and taken out from the waveguide 8. All the stubs 21-23 are simultaneously moved in the same direction and stopped in the position where the reflected power detected by the reflected power detector 10 is a minimum value, the respective stub 21-23 are successively moved one by one and stopped in the positions where the reflected power detected by the reflected power detector 10 is the minimum value, respectively.