coupling hole 44a in fig9 numeral 41 designates a housing in which a portion of a flange plate 42 is fixed the buckle 4 is secured by the flange plate 42 to the housing 41 near the floor of the vehicle not shown in the housing 41 a gas generator 43 as the gas generating means is fixed to the flange plate 42 the gas generator 43 is firmly connected to a buckle body 44 acting also as a gas passing pipe fig9 shows a state of coupling the tongue pipe 33 to the coupling hole 44a as shown in the figure when the tongue pipe 33 is coupled to the coupling hole 44a of the buckle body 44 a tongue pipe releasing ring 45 is pressed against a biasing force of the spring 46 by the tongue pipe distal end 33a in a direction of the arrow a while tapered iadd tapered iaddend holes 44b are formed at a peripheral surface of the buckle body 44 at predetermined intervals the metal ball 47 is inserted in each tapered hole 44b to be able to move from the peripheral surface side of the buckle body 44 so that a portion of the metal ball 47 projects from the inner surface of the coupling hole 44a when the tongue pipe 33 is coupled to the coupling hole 44a each of the metal balls 47 is pressed from the peripheral surface side of the buckle body by a release operation ring 49 biased by a spring 48 in a direction of the arrow b therefore.