PURPOSE: To prevent vane friction due to slip and to improve frictional resistance of the outer surface by providing a sleeve rotatably in the housing, while said sleeve being made of steel material and inner and uter surfaces of said sleeve being coated with nitride coating and the inner surface having surface roughness smaller than that of outer surface.

CONSTITUTION: The rotary sleeve is made of steel material. The inner and outer surfaces 12, 17 of said sleeve are coated with a film of nitride formed through nitrification process. Surface roughness of inner circumference 12 is considerably large and the surface roughness of outer circumference is made smaller than that of outer surface 17. The surface film structure along the inner face of rotary sleeve 5 comprises pearlite 21, butt graphite 22 and nitride film 23. Projection 23a is formed in nitride film 23 around the periphery of graphite 22.