To provide a method, by which a prescribed element level in the atmosphere in an enclosure is efficiently adjusted to in prescribed range.

A first gas source 1 and a second gas source 2 are used. The element level in the first gas source 1 is higher than the second gas source 2. Then, under the nominal condition, the first gas source 1 is used to supply the nominal flow rate Qn into the enclosure 3. When this element level in the atmosphere in the enclosure 3 is larger than a first upper threshold value (Lh-), the supply of gas from the first gas source 1 into the enclosure 3 is shut off and the gas from the second gas source 2 is supplied into the enclosure 3. The supply under the nominal condition into the enclosure 3 is received only in the case the measured result of the element level in the atmosphere is smaller than the second upper threshold value (Lh-α).