PURPOSE: To contrive sharp reduction of manufacturing cost in such a way that an impeller is formed by means of a mold using inexpensive resin in which silicate for alleviating heat strain is mixed in its proper rate and, ring-shaped ribs of the impeller are formed by means of a mold integrally with blades.

CONSTITUTION: At the same time as mold formation is done by using inexpensive resin which is made by mixing calcium silicate in approximately 20% of weight ratio for alleviating heat strain in polypropylene in which denatured polymer and copolymer are mixed almost in the same rate, which has larger heat strain than polycabonate does, in the contrivance to form a plurality of blades which stretches in the axial direction on the identical circumferential plane, ring-shaped ribs 2, 4 which surround the outer periphery of said blades at the intermediate position in the longitudinal direction of blades 1 are integrally formed with the blades 1 by means of a mold. Thus, the fan's cost can be sharply reduced by using more inexpensive resin than polycarbonate.