In an internal combustion engine including a driving-sprocket cam chain guide 70 for guiding a cam chain 47 along a driving sprocket 45, as viewed in a crankshaft axial direction, a guide body 71 of the driving-sprocket cam chain guide 70 extends beyond a winding termination point P of the cam chain 47 on the driving sprocket 45 to a position adjacent to a chain moving surface 51c of a cam chain tensioner 51, on which the cam chain 47 slides. The engine provides a cam-chain dropout preventing function to the cam chain guide, thus reducing load on the cam chain tensioner and improving durability of the tensioner lifter.