PURPOSE: To enable a round hole to correspond to the position displacement of a protrusion in accordance with a tilt of a return vane, by deflecting the center of the round hole, drilled in a separator plate, in a size larger than that of said protrusion for the return vane and the center of the vertically provided protrusion on a straight line passing through the center of a shaft hole and the protrusion.

CONSTITUTION: For the center of a positioning protrusion 12 vertically provided in a return vane 3-2, the center of a positioning round hole 14, drilled in a separator plate 3-3', in a size larger than that of said protrusion 12 is set to a position displaced by the dimension D to the center side of a shaft hole 11 provided in the separator plate 3-3'. In this way, the round hole can sufficiently also correspond to the position displacement of the protrusion 12 when the return vane 3-2 is tilted by contraction or the like generated in accordance with cooling resin after molding, and the positioning protrusion 12 of the vane 3-2 can be fitted to the positioning round hole 14 of the separator plate 3-3'. Further concentricity of the separator plate 3-3' with a shaft 9 and a gap between an end face of the shaft hole 11, provided in the separator plate 3-3', and the periphery of the shaft 9 can be easily ensured.