PURPOSE: To provide an impeller that is manufacturable so well in quality and accuracy in addition to being inexpensive in cost, without using plural units of forming dies, in such consisting of a main plate, a side plate and intermediate plate held between these plates aforesaid.

CONSTITUTION: Plural pieces of arced slide cores 20 are installed on a lower die 16 at regular intervals, and a shaft 22 is connected to this slide core 20. A first gear 23 and a second gear 24 converting rectilinear motion into rotational motion are connected to this shaft 22, and out of them, the second gear 24 is connected to a rack pinion 27 being driven by movements of an upper die 25 to be joined with the lower dies 16.