In a buckle of the present invention, inertia force F.sub.1 acts in a direction opposite to a direction toward a disengaging position of a main body 4d of a lock member when the buckle is pulled by a pretensioner. Clockwise torque T.sub.1 is developed on a lever 12b and counter-clockwise torque T.sub.2 is developed on a mass body 12c. Since the torque T.sub.2 of the mass body 12c is larger generally, the lever 12b and the mass body 12c rotate together in the counter-clockwise direction so that the lever is set in the restricting position. At the bottoming of the pulling action of the buckle, the inertia force F.sub.2 acts in a direction toward the disengaging position of the main body 4d. Therefore, the mass body 12c rotates in the clockwise direction and the lever 12b is held in the restricting position so that the movement of the lock member in the direction toward the disengaging position of the main body 4d is prevented. As a result of this, unexpected disengagement between the buckle and the tongue is prevented even with extreme acceleration, the operational feeling of an operational button is improved, interchangeability is provided to the operational button, and the assembly condition of components such as the operational button is improved.