PURPOSE: To rapidly and accurately detect the leakage of a gas by sealing the opening and closing part of the airtight vessel of a vapor growth device with two O rings, purging the gap between the two O rings, with an inert gas, sucking the gas with a vacuum pump, and introducing the gas to a gas detector.

CONSTITUTION: In the vapor growth device using a harmful gas, the outer O ring 8 and the inner O ring 9 are provided between the rim 1b of the opening of a bell jar 1 and a base plate 7. When the device is operated under such conditions, an inert gas (especially gaseous N2) is passed between the O rings 8 and 9 from a pipeline system 19, and sucked by a vacuum pump 11 through a gas detector 10. As a result, a trace amt. of the harmful gas escaping from the inner O ring 9 is not leaked to the outside from the outer O ring 8, flows to the gas detector 10 along with the inert gas, and can be rapidly and accurately detected. Safety is easily monitored in this way.