Impeller manufacturing method and impeller

An impeller comprising:

• a base having a first annular surface with an annular shape in plan view;
• a shroud provided at a distance from the base in a direction of a central axis, and having a second annular surface with the same central axis as the central axis of the base and an inner diameter larger than an inner diameter of the first annular surface in bottom view;
• and
• a main blade portion having a plurality of thin-plate-like upright portions provided between the first annular surface and the second annular surface and extending in a direction parallel to the central axis and in a radial direction, and a plurality of thin-plate-like inclined portions formed so as to be continuous with the upright portions and extending in the radial direction further inside than an inner circumferential surface of the shroud while being inclined with respect to the central axis, the inclined portions not overlapping each other in plan view.