To provide an efficient type dust discharge fan generating low noise.

In this dust discharge fan, a duct having nearly reverse S-shaped cross section and rotatably horizontally and internally installing an impeller 35 in the lower curved part B is formed and the space between the inside diameter of a cover 41 for covering the impeller 35 in close contact along a part of the outer periphery of the impeller 35 making the curved part B on the lower side of the duct and the outside diameter of the impeller 35 is made to be proportional to the outside diameter D of the impeller 35 and the proportional coefficient is set to 0.08±0.02.