An impeller in a water pump for circulation of cooling medium in a cooling system of an engine has a shroud with a shroud end to which the edges of the blade inlets are attached and which is formed in the cylindrical configuration substantially parallel to the rotary shaft. Each of the edges of the blade inlets is shaped so that it is continuously smooth from a shroud end surface at the inlet side thereof and extends upstream in the axial direction, while each of the edges of the blade inlets at the side of a casing extends substantially perpendicularly to the rotary shaft. The edge of the blade inlet attached to the cylindrical shroud end and the edge of the blade inlet at the casing side being connected therebetween by a smooth arc-like curve projecting convexly upstream. The inlet angle of the blade is set to substantially 0.degree. at the inlet edge at the shroud end and to an angle calculated substantially on the basis of the conventional design at the inlet edge at the casing side. The impeller has the blades each formed by connecting the blade inlet to a blade outlet by a smooth curved surface.