A variable valve timing control system of an internal combustion engine includes a hydraulically-operated phase converter disposed between a sprocket and a camshaft, and having a phase-advance hydraulic chamber and a phase-retard hydraulic chamber for changing an angular phase of the camshaft relative to the sprocket. An electric pump is provided to supply working fluid selectively to one of the hydraulic chambers via a directional control valve. Also provided is a check valve disposed in a discharge line of the pump for permitting flow in a direction that the working fluid flows from the pump to the directional control valve and preventing any flow in the opposite direction, so as to prevent a pulse pressure arising from alternating torque exerted on the camshaft from being transmitted from either one of the hydraulic chambers via the discharge line to a discharge port of the pump.