To appropriately cool oil in simple constitution by interposing an oil cooler on the way of a cooling water discharge pipe, in the cooling device of a marine engine in which water outside a hull is taken in as cooling water and fed to the engine, and discharged outward through the cooling water discharge pipe after cooling the engine.

This engine 100 driving a jet propulsion machine is provided with a first cooling water pipe 11 introducing cooling water taken from the nozzle of the jet propulsion machine on the stern part, and the extreme end part of the cooling water pipe 11 is connected to the inlet of a cooling water passage formed on an exhaust manifold 13 through cooling water feed pipes 12. Cooling water from the exhaust manifold 13 enters a cylinder head 19 through a cooling water pipe 15, the cooling water therefrom is fed to the cooling water passage 43 of an exhaust pipe 41 of douplex pipe construction. Thereafter the cooling water is fed to an oil cooler 51 through a cooling water pipe 25, and the cooling water discharged from the oil cooler 51 is discharged outside the ship through a cooling water pipe.