PURPOSE: To improve accuracy of dynamic balance by simplifying work for securing a balance weight.

CONSTITUTION: A boss 3 is mounted on a small diametric part of a circular truncated cone-shaped hub 2 formed of a thin plate by securing a plurality of blades 1 to the periphery. A casing 4 is provided in an outer side of the blade 1 through a gap. A blower is driven by an electric motor through a shaft, not shown, in a hole 3a of the boss 3, and this electric motor is mounted on the casing 14 by a stay not shown. A small diametric annular part 11 facing to the inside in the axial direction is provided in a small diametric part of the hub 2, and a U-shaped balance weight 12 can be secured so as to be calked astride to an end part of this small diametric annular part 11. A large diametric annular part 13 facing to the inside in the diametric direction is provided in a large diametric part of the hub 3, and a U-shaped balance weight 14 can be secured so as to be calked astride to an end part of this large diametric annular part 13.