A rotor unit comprises an inner rotor and an outer rotor having different number of teeth, with the inner and the outer rotor being mounted on eccentric shafts to be driven for rotation at different speeds. The volume efficiency is improved by producing a clearance between the external surface of the outer rotor and the inner surface of the housing so that the clearance changes from the axial center to either end so that the magnitude of the clearance is reduced during a high speed rotation when a centrifugal force is effective. Projections are formed on the opposite ends of the outer rotor to reduce the volume of a waste space which is defined between the inner and the outer rotor.