To provide a propulsion of a ship, by restraining the generation of cavitation.

An outside propeller 3 having a plurality of blades 7 and an outside ring member 8 forming a tooth 8a on the inner periphery at the inner end of the blades 7, is arranged on the outer peripheral side adjacent position of an inside propeller 2, on the outside of an inside propeller 2 having a plurality of blades 5 installed on the propeller central shaft 1 and an inside ring member 6 forming a tooth 6a on the outer periphery at the outer end of the blades 5. A gear 4 meshing with the respective teeth 6a and 8a is arranged between the respective ring members 6 and 8. When rotating the inside propeller 2, the outside propeller 3 is rotated in the opposite direction of the inside propeller 2 by the gear 4, and a vortex generated in a blade tip of the inside propeller 2 and a blade root of the outside propeller 3 is cancelled, and a vortex generated in a blade root of the inside propeller 2 and a blade tip of the outside propeller 3 is weakened, controlling the generation of the cavitation.