PURPOSE: To prevent a burning damage of the blade of a prompt breaking valve by closing the rapid shut-off valve after a high-frequency wave oscillator device is stopped, when a vacuum breakdown trouble is generated.

CONSTITUTION: When a vacuum breakdown trouble is generated in the BL-3, the output signal 11-1a of a system controller 11 outputs an RF OFF instruction 14a to stop a high-frequency wave oscillator device 6 from a transmitter 14, and the power of a high-frequency accelerator hollow body 5 is stopped. The condition signal 5a that the RF power is OFF enters a receiver 15, it is converted from series to parallel, and the signal 15-3a of the BL-3 is outputted and inputted to an AND circuit 16. In the AND circuit 16, only when the AND condition of the vacuum deterioration signal 10 of the vacuum breakdown trouble and the signal 15-3a is established, the output is issued to drive the valve driver 11-3 of the system controller 11, and a rapid shut-off valve 12 is closed. Consequently, since the high-frequency wave accelerator hollow body 5 is being stopped, no influence from an undulator device 4 is given and a burning damage of the blade of the rapid shut-off valve 12 can be prevented.