An overhead-camshaft internal combustion engine having an outer wall of a crankcase provided adjacent a crankshaft, with a spiral-surfaced opening through which the crankshaft is passed. The outer crankcase wall is integrally provided with a loop guide engaging recess formed thereon. The loop guide engaging recess supports a base end portion of a loop guide for guiding an endless chain. A loop-retaining member for preventing the timing endless chain from dropping is arranged at a position on the lower side of a loop drive sprocket integrally formed at an end portion of the crankshaft.