PURPOSE: To prevent carbon from accumulation in a gap between an air casing, joined with a rotor housing containing a rotor having a number of cell spaces partitioned by a cell wall, and a rotor, by a method wherein air is exhausted and fed in the gap.

CONSTITUTION: A rotor 3, provided in its outer periphery with a number of cell spaces 7 partitioned by cell walls 6, is rotatably contained in a rotor housing 1, with the one side of which an air casing 2 is joined. After the rotor 3 pressurizes air, introduced through a primary intake air port, the air is exhausted through a secondary intake air port 9. In this case, an air introduction passage 20 is formed throughout a range of from the air casing 2 to an outer wheel 11a of a bearing 11, its inner end is open to a bearing chamber 5 of the rotor 3, and is communicated to a gap S between the air casing 2 and the rotor 3 through a gap between the inner wall surface of a bearing chamber 5 and the outer peripheral surface of the outer wheel 11a. Plural fins 21 are formed in the inner wall surface of the bearing chamber 5 in a manner to be positioned facing an opening part 20a on the bearing chamber 5 side of the air introduction passage 20.