PURPOSE: To prevent entrance of solid-gas-fuel mixture to a space, formed by using the casing, the impeller, the main plate, and the side plate of a centrifugal compressor, without increasing a blowing amount of buffer gas.

CONSTITUTION: By mounting spiral fins 7 and 8, the centers of which are the rotation center of an impeller 3, on the main plate 3b of the impeller 3 of a centrifugal compressor 1 and a side, ward positioned opposite to a side plate 3c, the outward air current of the impeller 3 is generated through the working of the spiral fins 7 and 8 rotted along with rotation of the impeller 3. Since the direction of the flow of solid-gas-fuel mixture Gp about to enter spaces 4 and 5 is varied, entrance of the solid-gas-fuel mixture Gp to the spaces 4 and 5 is prevented from occurring without increasing a blowing amount of buffer gas Gb fed through buffer gas feed holes 9 and 10.