To provide a fume hood capable of safely conducting discharge without increasing the whole exhaust air volume by confining the internal air without leakage to an indoor space such as a laboratory even if the internal air is turned into the positive pressure atmosphere due to the high quantity of heat produced by the source of heat generation within a treating chamber of the hood.

In the fume hood 11 equipped with the hood itself 13 formed so that the expansion and shrinking of the opened area of the front opening 16 of the treating chamber 15 may be free in relation with the opening of an elevating sash window 17, an air-feeding chamber 33, which uptakes the air-conditioned air in the indoor space such as a laboratory and blows the air downward compulsorily so that the air runs along the elevating sash window 17 as the confined air 45, is installed at the ceiling part 14 side of the hood itself 13 located at the upper position of the elevating sash window 17. Thus, the internal air in the treating chamber 15 when it is turned into the positive pressure atmosphere due to the high quantity of heat from the source of the heat generation, is prevented from leaking into the indoor space side by the force of confinement produced by the confined air 45, and can be exhausted to the outside.