There is provided a buckle including a plug and a socket. The plug includes a plug-side base part, a pair of leg parts, and a pair of engagement protrusions. The socket includes a socket-side base part, an upper plate, a lower plate, upper side wall parts, lower side wall parts, engagement stepped portions, slits, and guide surfaces. A dimension of the pair of engagement protrusions in the upward and downward direction is set to be greater than a dimension of each of the slits in the width direction.