Based on this proposal, the plurality associated with vanes formed concerning the outer circumferential surface on the impeller on the axial fans is consistently moulded to make below servings or back side servings. These extend towards the central part of the base part of the impeller. Consequently, the slide bit on the mould that constitutes the "undermoulding portions" for all those vanes ought to be pulled out on the central part of the base part of the impeller in parallel for the radial path. As an effect, the mould can not become complex. Accordingly, the mould becomes really expensive.