To provide an inexpensive resinous impeller by manufacturing the impeller by joining components that can be each formed by an ordinary mold configured by combining a core and a cavity without using a slide core. An impeller is manufactured by: a step for forming, by molding, a first component which includes a base part having a first annular surface with an annular shape in plan view; a step for forming, by molding, a second component which includes a shroud having a second annular surface with an annular shape with an inner diameter larger than the inner diameter of the first annular surface in bottom view, and a plurality of thin plate-shaped main blade parts each having an inclined section which extend only radially inside the inner peripheral surface of the shroud while being inclined with respect to the axis of the shroud and do not overlap each other in plan view; and a step for joining the first component and the second component while the first annular surface and the second annular surface face each other.