PURPOSE: To change the characteristic of a fan and to ensure a suitable air volume by providing an orifice in which a plurality of plates arranged in a rink-like shape in a gas discharge passage of an impeller, are linked together so as to rise and fall for changing the flow passage area.

CONSTITUTION: When a worm screw 8 of an orifice 4 in an impeller 2 is rotated, pinions 6 are rotated. Damper plates 5 are fixed to the pinions 6 so that the directions thereof are changed in association with the rotation of the pinions 6 so that they rise and fall. The orifice 4 is attached thereto with several pinions 6 and plates 5, and the plates 5 are associated with one another by an outer ring 7 so that the directions thereof are changed simultaneously. The shape of each blade is curved so as to adjust the flow passage area of the fan in accordance with the change. Thereby it is possible to change the characteristic of the fan.