PURPOSE: To eliminate the formation of a closed-loop and improve the noise resistance by providing an AD converter for converting the signal to the digital signal and an insulating unit in a signal line for feeding back the signal from a monitor of a high frequency acceleration cavity to a high frequency power source.

CONSTITUTION: The signal output from a high frequency power source 1 is applied to a high frequency acceleration cavity 2 through an output line 3. Next, the analog signal of a monitor 4 is converted to the digital signal with an A/D converter, and thereafter, the digital signal is insulated 7 and fed back to the high frequency power source 1. Consequently, the formation of a circular closed-loop is eliminated with the output signal line 3 from the high frequency power source 1 to the high frequency acceleration cavity 2 and a feed back signal line 5 from the monitor 4 to the high frequency power source 1.