A valve timing adjusting device has a lock mechanism for restricting a rotational motion of an internal rotor 7 relative to an external rotor 5. The lock mechanism has a plunger 45, which is movable along a radial direction of the external rotor 5 and restricts the rotational motion of the internal rotor 7 relative to the external rotor 5 on condition that the prevention of the rotational motion of the internal rotor 7 relative to the external rotor 5 is removable, a receiving hole 7a, which is arranged on an outer circumferential surface of the internal rotor 7 and receives the plunger 45, a spring 21, which presses the plunger 45 inwardly along the radial direction of the external rotor 5 by a prescribed pressure, a holder 46, which determines a position of the spring 21, and a knock pin 47 which prevents the holder 46 from being moved outwardly along the radial direction of the external rotor 5.