Abstract: An internal combustion engine having its cylinders tilted at an angle of less an than 90 degrees relative to a vertical axis is provided with a oil drain mechanism for drainign excess oil that collects when the engine stops. For this purpose, an oil drain hole, which communicates with an interior of the piston body to drain the lubricating oil, is formed in the inner wall of an oil ring groove in such a way as to be located below the center line of the piston body. Both ends of the oil drain hole are set lower than the lowest portion of a piston head. Therefore, it is possible to prevent lubricating oil from entering the combustion chamber when the engine stops, without providing a whirl-stop on piston rings.