PURPOSE: To surely prevent the leak of exhaust from a part between joint surfaces by installing a sealing member for sealing the part between the joint surfaces of a joint part onto a part which surrounds an exhaust passage in the joint part between the upper and lower cases which built in a power transmission device for transmitting the engine output to a propeller.

CONSTITUTION: The propulsion unit 12 of an outboard motor 10 is constituted of a casing 13 and an engine 14, and the casing 13 is formed by joining an upper case 15 and a lower case 16 each other. A power transmission device for transmitting the revolution of a drive shaft 18 to a propeller shaft 19 is built in the casing 13, and an exhaust passage 24 is extended in the vertical direction. The exhaust of the engine is discharged into the outside water through an exhaust expansion chamber 28, exhaust passage 24, and an exhaust passage 30 from an exhaust pipe 27. In this case, a sealing member 36 for sealing the part between the joint surfaces of the joint part is installed onto a part which surrounds the exhaust passage 24 in the joint part of the upper and lower cases 15 and 16 of the casing 13.