A buckle device having a buckle main body configured to releasably receive a tongue plate connected to webbing, the buckle main body comprising: a frame member; a slide button provided slidably on one side of the frame member; the slide button comprising an operation portion in a plate shape, and a pair of side plates extended in parallel with each other from both lengthwise ends of the operation portion in a slide button sliding direction and installed to hold an upper plate of the frame member widthwise therebetween from outside; a base member fixedly provided on the frame member so as to be positioned above the slide button with a side facing the operation portion; a latch member provided in the base member, movably in a direction perpendicular to the frame member, the latch member configured to be engaged with the tongue plate when the tongue plate is inserted, and to be disengaged with the tongue plate by pressing the operation portion of the slide button and slidingly moving the slide button; a stopper member comprising an inertial mass body whose gravity center is eccentric toward the frame member from a rotation axial core; the stopper member further provided with an end portion facing the operation portion, rotatably in a direction toward the frame member, characterized by the stopper member provided with a pair of stopper rotating shafts projecting to both sides widthwise outward with respect to the frame member, the pair of stopper rotating shafts being pivotally supported by a pair of stopper supporting pieces extended from the side of the base member facing the operation portion in parallel with each other toward the operation portion, the stopper member configured to stop movement of the slide button when the end portion of the stopper member facing the operation portion is rotated toward the frame member; and an urging member configured to urge the end portion of the stopper member facing the operation portion to rotate in a direction away from the frame member, wherein, when the buckle main body with the tongue plate inserted therein experiences abrupt acceleration in a direction to tighten the webbing, an end portion of the inertial mass body at a side of the operation portion is rotated toward the frame member by an inertial force acting on the inertial mass body against an urging force of the urging member and stops the movement of the slide button slidingly moved by an inertial force acting on the slide button, preventing disengagement of the latch member and the tongue plate. a frame member; a slide button provided slidably on one side of the frame member; the slide button comprising an operation portion in a plate shape, and a pair of side plates extended in parallel with each other from both lengthwise ends of the operation portion in a slide button sliding direction and installed to hold an upper plate of the frame member widthwise therebetween from outside; a base member fixedly provided on the frame member so as to be positioned above the slide button with a side facing the operation portion; a latch member provided in the base member, movably in a direction perpendicular to the frame member, the latch member configured to be engaged with the tongue plate when the tongue plate is inserted, and to be disengaged with the tongue plate by pressing the operation portion of the slide button and slidingly moving the slide button; a stopper member comprising an inertial mass body whose gravity center is eccentric toward the frame member from a rotation axial core; the stopper member further provided with an end portion facing the operation portion, rotatably in a direction toward the frame member, characterized by the stopper member provided with a pair of stopper rotating shafts projecting to both sides widthwise outward with respect to the frame member, the pair of stopper rotating shafts being pivotally supported by a pair of stopper supporting pieces extended from the side of the base member facing the operation portion in parallel with each other toward the operation portion, the stopper member configured to stop movement of the slide button when the end portion of the stopper member facing the operation portion is rotated toward the frame member; and an urging member configured to urge the end portion of the stopper member facing the operation portion to rotate in a direction away from the frame member, wherein, when the buckle main body with the tongue plate inserted therein experiences abrupt acceleration in a direction to tighten the webbing, an end portion of the inertial mass body at a side of the operation portion is rotated toward the frame member by an inertial force acting on the inertial mass body against an urging force of the urging member and stops the movement of the slide button slidingly moved by an inertial force acting on the slide button, preventing disengagement of the latch member and the tongue plate.