PURPOSE: To prohibit generation of distortion in a rib caused by centrifugal force by extending a reinforcement part along the whole length of the rib, and extending it only from an inflow end part of one rib to an inflow end of the other rib to have no effects on inflow cross sectional surface of a carrying chamber formed of the two ribs.

CONSTITUTION: Refinforcement parts 32, 32 are provided at outer circumferential parts of ribs 14, 14, and the reinforcement parts 32, 32 are provided on outer walls of the ribs 14, 14. They are extended along the whole length of the ribs 14, 14 in the axial directions at least, and extended only from an inflow end part of one rib 14 of the plural ribs 14 to an inflow end part of the other rib 14 continuous with above rib 14 in the circumferential direction. In this case, they will not have effects on an inflow cross sectional surface of a carrying chamber 15 formed of the two ribs 14, 14. Generation of distortion in the ribs 14, 14 by centrifugal force can be largely prohibited by the reinforcement parts 32, 32.