A plurality of recesses, each having a semicircular section, are formed close to each other in the inner surface of a cylindrical housing so that these recesses extend parallel to the axis of the housing. The inner surface of the housing, a portion of a circle which is part of each recess, and the ends of the vanes of a rotor are provided starting from the curvature centers of the semicircular recesses to the axial center of the housing.