PURPOSE: To improve performance and to reduce the generation of noise by a method wherein by equalizing an angle of a blade warp line with a line segment, the warp height of the blade is set to a value higher than that of a special formation as an inlet angle and an outlet angle are kept at an optimum value.

CONSTITUTION: A mixed flow impeller 11 is provided with a plurality of blades 15 disposed to the periphery of a hub 12 in a conical trapezoid. A cross line between an approximately conical surface, on which the hub 12 coincide with a central axis, and the blade 15 is considered on the development surface of the approximately conical surface, and the one end of the two ends of the cross line is A, the other end is B, the length of a line segment AB is l, and a maximum distance between the cross line on the development surface and the segment line AB is L. Provided an angle of the cross line and the segment line AB at a point A is ξ1, an angle of the cross line with the segment line AB at a point B is ξ2, adjoining point, indicating the maximum distance L satisfying ξ1=ξ2 and L>l (sin-1ξ1-cotξ1/2), and the points A and B are intercoupled through a tertiary curve. This constitution sharply reduces the generation of noise.