An intake- and/or exhaust-valve timing control system for an internal combustion engine comprises an inner sleeve having an outer toothed portion, firmly connected to a front end of a camshaft, a cam sprocket having an inner toothed portion, connected through a timing chain to an engine crankshaft, and a ring gear mechanism including inner and outer toothed portions respectively meshing with the outer toothed portion of the sleeve and the inner toothed portion of the cam sprocket, for changing the intake and/or exhaust-valve timing of the engine. The valve timing control system also includes a knock-pin provided on the front end of the camshaft at a predetermined angular position of the camshaft and a positioning hole formed in the sleeve, for press-fitting the knock-pin thereinto, such that the location of the positioning hole determines the relative phase angle relationship between the knock-pin and a particular point on the outer cylindrical member which defines a reference angular position of the outer cylindrical member. Further provided is an adjusting screw, which engages one end of the ring gear member to provide axial sliding movement of the ring gear member, for fine adjusting the relative phase angle between the sleeve and the cam sprocket after installation of the ring gear member between the sleeve and the cam sprocket.