a buckle that includes a buckle body that engages with a tongue plate inserted through an intermediate portion of a webbing belt for restraining a vehicle occupant, and an elongated buckle stay having a proximal end connected to the buckle body;

a pulling device that is coupled to the buckle and actuated when the vehicle undergoes a sudden deceleration, with the pulling device pulling the buckle in a vehicle occupant restraining direction of the webbing belt; and

a guide device which is pivotally connected to a distal end of the buckle stay and, due to the actuation of the pulling device, pivotally rotates the buckle body from an engagement position where the buckle body engages with the tongue plate at an ordinary time, through an intermediate position further toward a downward direction of the vehicle than the engagement position, and axially withdraws the distal end of the buckle stay to further position the buckle body to a lumbar region-corresponding final position that is positioned further toward a rearward direction of the vehicle than the intermediate position in the downward direction of the vehicle and corresponds to a lumbar region of a seated vehicle occupant,

wherein the guide device includes a rail that corresponds to a moving path of the buckle body from the engagement position to the final lumbar region-corresponding position and is curved and formed in a substantial “L” shape, with the curved portion corresponding to the intermediate position of the webbing belt and being disposed further in the downward direction of the vehicle than the engagement position, and a retaining member that retains the buckle stay in the rail so that the buckle stay is movable along the rail along a substantially elliptical curve.