An intermediate lock mechanism is formed by first and second lock pins, an advancement restricting groove, and a retardation restricting groove. The first and second lock pins are arranged in a vane rotor 2 and projectable and retractable independently from each other. The advancement restricting groove is formed in a cover and becomes engaged with the first lock pin when the first lock pin is projected, thereby locking, at an intermediate lock phase, rotation of the vane rotor to the advancing side. The retardation restricting groove is formed in a cam sprocket and becomes engaged with the second lock pin when the second lock pin is projected, thereby locking, at the intermediate lock phase, rotation of the vane rotor to the retarding side. In this manner, reliable locking is ensured, and chattering of the vane rotor is easily prevented when locking is performed.