PURPOSE: To facilitate molding without lowering the characteristic of an impeller by setting a number of vanes in such a relation that the adjacent vanes are not mutually superposed in the axial direction, forming the inside and outside portions of each vane into specific shapes, and forming a motor cover in a curved shaped.

CONSTITUTION: An impeller 6 is formed with a spherical boss portion 16 in which a rotary shaft 5 is inserted in the center portion thereof, and a number of vanes 17 which are projectingly provided on the peripheral wall portion of the boss portion 16. In this case, the spherical face of the boss portion 16 is formed into a curved face which continues to the spherical face of a motor cover 15. On the other hand, the vanes 17 are formed in such a relation that the adjacent vanes are not mutually superposed in the axial direction. The inside portion 17a of each vaned 17 which is superposed on the boss portion 16 from the upper curse side in the axial direction is perpendicularly provided in the axial direction without being inclined in the axial direction. The outside portion 17b of each vane 17 which is positioned on the outside of the outer peripheral edge of the boss portion 16 is inclined with respect to the axial direction. The whole vanes 17 are curved in a direction perpendicularly intersecting to the axis.