To prevent unnecessary increase in consumption of lubricating oil for an internal combustion engine, and to prevent a propulsion device from upsizing, when blow-by gas is burned by returning to an internal combustion engine through an intake air passage, in a propulsion device for a small boat.

Air 7 in the atmosphere side is sucked into the inside of an internal combustion engine 6 successively through an intake air passage 40 in an intake air box 34 and the intake air passage 40 in an intake air introducing member 31, and blow-by gas 57 generated in the internal combustion engine 6 is sucked into the intake air passage 40 in the intake air box 34 through a breather pipe 61. An oil separating chamber 59 is molded by dividing a part of the intake air passage 40 in the intake air box 34. An extending end portion 60 of the breather pipe 61 is opened to the oil separating chamber 59, and a gas exhaust passage 63 for making the oil separating chamber 59 communicate to the intake air passage 40 in the outside of the oil separating chamber 59 and within the intake air box 34.