To improve the service life of an outboard engine, and to maintain the excellent appearance by effectively cooling a part to be easily heated by the heat of the exhaust gas from an internal combustion engine passing through an exhaust passage provided inside a case of the outboard engine.

This device for cooling an external surface of a case of an outboard engine is provided with a case 9 in which an outboard engine 4 is supported by a hull 3 and which is extended in the vertical direction and a lower part of which is submerged below the water 2 level, an internal combustion engine 10 to be supported by an upper end face of the case 9, a propeller to be turnably supported by a lower end part of the case 9, a power transmitting means 12 which is provided inside the case 9 and connects the propeller 11 to the internal combustion engine 10 in an interlocking manner, and an exhaust passage 15 which is provided inside the case 9 and guides the exhaust gas 14 to be discharged from the internal combustion engine 10 outward of the lower end part of the case 9. A water feeding means 48 to discharge and feed the water 2 outside the hull 3 to an external surface of a part to be heated 45 opposite to the exhaust passage 15 in the horizontal direction out of the upper part of the case 9, is provided.