PURPOSE: To reduce the quantity of liquid nitrogen used by arranging a degassing heater on the atmosphere side of the vacuum container, a cooling container cooling a vacuum container, and a refrigerator feeding the cooling gas to the cooling container.

CONSTITUTION: When the degassing process in a vacuum container 1 is completed as the prepreparation, liquid helium 4 is transferred into a refrigerant tank 5, and liquid nitrogen 8 is transferred into a liquid nitrogen container 7. Next, the cooling air A from a refrigerator 17 is fed into a cooling container 16 and circulated to cool the vacuum container 1. After the container 1 is thus evacuated, even if helium gas is guided into an ion generating section 2 and the degree of vacuum in the container 1 is abruptly lowered, the wall face of the container 1 is kept cooled, and the temperature of the liquid nitrogen storage container 7 is not increased. Accordingly, the liquid nitrogen 8 stored in the container 7 is not decreased, and the quantity of the liquid nitrogen used can be reduced.