PURPOSE: To reduce a noise and required power by providing a specific shaped ring shroud at the round opening of a plenum chamber.

CONSTITUTION: A ring shroud having its upper and lower ends flared out in funnel like shape is provided around the outer end of fan blades 7 of a fan 5 installed in a round opening 3 of a plenum chamber 4 with said flared portion overlapping the periphery of the round opening 3, whereby the rotation of the fan 5 in the direction of an arrow 9 causes an air flow 20 at the entrance near the outer end of the fan 5 to flow smoothly due to gradual reduction of the flow passage by the ring shroud 8 and also causes a flow 21 after the air presses through the fan 5 to flow smoothly due to gradual increase of the low passage, and the presence of an overlapped portion 11 creates smooth flow line in the air flow within the plenum chamber 4 to reduce the noise and required power.