PURPOSE: To improve the productivity by injection moulding an open impeller of blades and a hub having a diagonal surface of thermoplastic synthetic resin, and heating ranges from the root of the blades to their tip for softening and bending them circularly.

CONSTITUTION: For manufacturing diagonal impeller, an open impeller consisting of a plurality of blades 12 having a two-dimensional form perpendicular to a surface crossing the axial center and a hub 11 having a diagonal surface is injection molded of thermoplastic synthetic resin first. A heater 13 is then provided at a blade installing part of the hub 11, a heat insulating part 14 is applied to the other part, it is heated around its root 12c at about the softening point of the synthetic resin by the heater 13, and a force Q is applied in this condition to deform the blades 12 to be like 22. In this case, a jig is applied to the back side of the blade 22 as necessary for keeping the bending form properly, and it is then cooled in this condition to obtain a three- dimensional blade 22.