The present invention relates to a four-stroke cycle internal combustion engine in which lubrication oil is atomized to generate oil mist so as to lubricate an internal mechanism of the engine with the oil mist. The four-stroke cycle internal combustion engine includes an oil-mist breather in fluid communication with a fuel tank of the engine. Any oil mist remaining after the lubrication of the internal mechanism is discharged into the fuel tank 14 through the oil-mist breather 22. Since no oil mist remains in the oil-mist breather the air filter of the engine is not contained by the oil mist.