Abstract: A structure of internal combustion engine comprises a cylinder member composed of a cylinder which accomodates a piston connected with a crankshaft, a crankcase provided to accomodate said crankshaft, a rocker case provided to accomodate a valve actuating mechanism, an oil return passage constituting means for leading oil in said rocker case into said crankcase, and a restraining means for restraining an air flow within said crankcase from entering into said oil return passage constituting means. By the effect of the restraining means provided in relation to the oil return passage constituting means, the air flow generated by the rotation of the crankshaft does not work as a dynamic pressure of hingering the flowingdown of oil.