An effective range of urging force of an assist spring is between a maximum retarded phase and a predetermined phase of a camshaft, a vane rotor and vanes. The predetermined phase is equal to an intermediate locking phase of the camshaft, the rotor and the vanes+10 degree CA. Even when oil pressure supplied to each advancing chamber is reduced at the time of engine stop, the camshaft, the rotor and the vanes can be advanced to or beyond the intermediate phase by the spring. Furthermore, at the time of engine start, the rotor and the vanes are positioned near the intermediate phase, so that reaction force of the spring is very small, allowing easy movement of the rotor with drive torque of the camshaft toward a retard side. Thus, the rotor can be locked at the intermediate phase by a lock pin.