A valve timing control system for accelerating activation of a catalytic converter in an internal combustion engine in a cold starting operation mode. The system includes actuators (15, 16) coupled to cam shafts (15C, 16C), hydraulic pressure supply units (19, 20) for driving the actuators (15; 16), and a controller (21A) for controlling a hydraulic pressure for the actuators (15, 16) in dependence on engine operation states to change phase of the cam shafts (15C; 16C) relative to a crank shaft. The actuator (15; 16) includes a locking mechanism (155, 157) for setting the relative phase to a lock-up position, and an unlocking mechanism (156) for releasing the locking mechanism in En response to a predetermined hydraulic pressure. The controller (21A) sets the relative phase to the lock-up position in a starting operation, while controlling the relative phase to be advanced in an engine state succeeding to the starting operation.