An internal combustion engine including an oil passage, which connects the interior of a crankcase and the interior of a cylinder head, is disclosed. The oil passage is open at the upper section of the crankcase. Oil in the cylinder head is transferred to the crankcase via the oil passage and trickles down along an inner wall surface of a skirt of the crankcase to be returned to an oil pan. The crankcase accommodates an operating member such as a connecting rod. A protrusion formed on the inner wall surface of the skirt extends substantially along a projected part, which is a projection of the operating member projected on the inner wall surface in a direction perpendicular to the axis of the crankshaft, such that the protrusion separates at least part of the projected part from a section of the inner wall surface closer to the opening of the oil passage.