When a pretensioner is activated, a buckle main body experiences abrupt acceleration in a direction toward webbing. An inertial amass body of a stopper member is then rotated toward a frame member against an urging force of a torsion spring urged through a lever member. Further, the lever member abutting on the stopper member is also rotated toward the frame member, making each projecting piece face each abutment portion of a slide button. Further, when the slide button is slidingly moved against an urging force of a button spring, each abutment portion makes contact with each projecting piece of the lever member, stopping the slide movement of the slide button. Thus, unintentional disengagement between a latch and a tongue plate can be prevented.