To satisfactorily confine contamination produced in an enclosure.

In this method, a contamination that is produced in at least one of an upper volume part 5 and a lower volume part 4 in an enclosure filled with a fluid is confined. The temperature of the upper volume part 5 is maintained at a temperature higher than that of the lower volume part 4 in only the temperature in which the space between the two volume parts can be securely separated from each other by a narrow turbulence intermediate area 3 in which a steep temperature gradient is maintained. A virtual confining barrier is formed where the turbulence intermediate area 3 functions as a virtual partition in a horizontal plane, and the contamination is confined in the volume parts by thermal stratification.