PURPOSE: To prevent reduction in any engine performance and to limit generation of such toxic exhausts as NOx and the like owing to security of a large amount of supplied intake air by providing a cooling water passage for cooling intake air in parallel to an intake pipe provided in an internal combustion engine used as an outboard engine.

CONSTITUTION: In a three-cylinder type vertical internal combustion engine disposed in the upper cowling of an outboard engine, a main bearing chamber 2 is fixed on the side of a cylinder block 3 integrally provided with a cylinder head 3h and cooling fins 2a are provided oppositely to the inner surface of this main bearing chamber 2. A cooling water passage 7 is integrally provided along an intake pipe 1 and an engine cooling water passage (k) is linked to the cooling water passage 7 via a cooling water communicating pipe 6. Sea water at a low temperature jetted out of a cooling water pump is first supplied to the cooling water passage 7 and a cooling water passage (a) and then supplied to the engine cooling water passage (k) of the cylinder head 3h through the cooling water communicating pipes 5 and 6. Thus, a temperature of intake air can be reduced and absorbing performance of the engine can be improved.