PURPOSE: To improve the hydrodynamic characteristics by providing a projected stripe at a site corresponding to the peripheral line of an impeller shape part in a forming die, arranging that a groove is formed in the peripheral line of the impeller shape part of a primary formed body, and making the cross section of the impeller to be formed to the shape approximated to the streamline shape.

CONSTITUTION: An impeller 11 for ventilating fan, etc., having multiple pieces of blades is made in three layer construction provided with a reinforced film layer such as polypropylene, etc., on both surfaces of a foaming resin layer. When an impeller 11 like this is manufactured, a sheet made of foaming resin is formed to a primary formed body 17 by a forming die 16. This primary formed body 17 is made to the shape provided with an impeller shape part 17a whereon the shape of the impeller 11 is carved in relief. Also, on both die surfaces 14a, 15a of the forming die 16, projected stripes 18, 19 are formed at the sites corresponding to the peripheral lines of the impeller shape part 17a to form grooves 20, 21 on both surfaces of the primary formed body 17. Using a cutting die, etc., it is cut along the base line of these grooves 20, 21, and other parts than the impeller shape part 17a are removed to form the impeller 11.