A driving force transmission system for an engine is provided with a crank sprocket mounted on a crankshaft, cam sprockets mounted on camshafts, an intermediate shaft disposed between the crankshaft and the camshafts, a dual sprocket mounted on the intermediate shaft, and including a first sprocket and a second sprocket facing each other in a state that the first and second sprockets are rotatable relative to each other, a first endless transmission member wound around the crank sprocket and the first sprocket, and a second endless transmission member wound around the cam sprocket and the second sprocket. The dual sprocket includes a damping portion for connecting the first and second sprockets. The damping portion is configured to exert a resilient force in a rotational direction of the first and second sprockets.