PURPOSE: To prevent moisture contained in the introduced atmospheric air from condensing on the outer wall surface of a liquefied gas tank, by providing a means for heating the outer wall of said tank to the room temperature in an apparatus in which a cold trap having therein a liquefied gas tank is attached to an intake system of a molecular turbo-pump.

CONSTITUTION: In effecting leakage test of an article A to be treated that is placed in a helium gas chamber D by using a vacuum exhausting apparatus in a leakage detector, a high vacuum is produced at first by operating a molecular turbo-pump 7, and then the temperature in a cold trap 1 is kept low.