An impeller for a water pump includes an impeller main body composed of a synthetic resin and having a vane member formed around a rotating center portion; and a metal boss in which a circular boss member positioned on the forward side of the rotating center portion and provided with a gradually narrowing portion at the axial end thereof, and a periaxial support boss member positioned on the rearward side of the rotating center portion are consecutively formed in the axial direction, and in which an axial support surface is formed between the circular boss member and periaxial support boss member, the metal boss being insert-fitted into said rotating center portion.