To prevent a white turbid substance generated due to contamination of water in lubricating oil from being deposited on a cap in a lubrication port structure of lubricating oil.

A lubrication port structure of lubricating oil includes a cylinder head cover for covering a cam sprocket provided to a cam shaft and a timing chain wound around the cam sprocket at the top of an engine; a cylindrical lubrication port in a position shifted to a side from an upper part of the timing chain in an upper wall part of a cylinder head cover; a projection projecting to a radially inside on an inner peripheral surface of the lubrication port; and a spiral groove fitted to the projection on an outer peripheral surface of the cap blocking the lubrication port. A tapered inclination surface in which an inner space expands toward a chain chamber side for accommodating the timing chain, is formed in a coupling part for connecting the lubrication port to a top wall part of the cylinder head cover. The inner space surrounded by the inclination surface is situated on an upper part of the timing chain.