Exhaust gas from an engine passes through a lower unit, and passes through a propeller boss coupled to a propeller shaft to be discharged into water. A stopper is provided at a rear end portion of a gear case of the lower unit, and a bush is inserted into and attached to an inner surface of the stopper. An inside diameter of the bush is set to substantially the same diameter as an outside diameter of a front end portion of the propeller boss, and the bush and the propeller boss are disposed by being overlapped with each other in an axial direction.