PURPOSE: To obtain a U-turn flow with a compact structure by making the inside of the hub of a diagonal flow impeller in a through state, and providing the first air passage member introducing a discharged flow to the central part of the bottom side of the hub, and return guide vanes.

CONSTITUTION: The inside of a hub 1b is made to be in a through state, and a plurality of return guide vanes 3 are formed on the bottom of the first air passage member 2 surrounding the circumference and bottom surface of a mixed flow impeller 1. One end of the second air passage member 4 of a cylindrical form is connected to the upper bottom outer circumference of the hub 1b. Thus, without adding a piping or the like to the outside of the mixed flow impeller.