An air-cooled internal combustion engine includes a cylinder block having a plurality of cylinder bores disposed in parallel to each other, a cylinder head secured to an upper portion of the cylinder block, a crankshaft, two camshafts disposed in a parallel, juxtaposed relationship to each other so as to extend perpendicularly to a forward and backward direction of a vehicle on which the engine is mounted at an upper portion of said cylinder head and positioned on front and rear sides of the engine relative to each other, and a camshaft driving mechanism for transmitting rotational driving force of the crankshaft to the camshafts. A cooling space into which wind can be introduced is formed below that one of the camshafts which is positioned on the front side of the engine. The entire cylinder head is thus cooled efficiently, including portions corresponding to the cylinders disposed at intermediate positions of the engine.