PURPOSE: To eliminate time required for vacuum re-startup at each adjustment by applying a magnetic fluid seal to connection with an acceleration cavity so as to rotate a high frequency electric power input part without breaking a vacuum.

CONSTITUTION: A magnetic fluid seal 5 is provided between a high frequency electric power input coupler 2 equipped with an antenna loop 3 on the top end thereof and an acceleration cavity main body 1. The high frequency electric power input coupler 2 is rotated without breaking a vacuum because of the magnetic fluid seal 5 part so as to conduct impedance matching of the cavity 1 with the high frequency electric power source, and when the impedance matching is performed fixing with a flange 6 is conducted. Time required for recovery to a vacuum after adjustment of a coupler 2 can thereby be eliminated.