passage 26 which allows the connection port 19 to communicate with the inside of the bag 12 is formed in the tongue housing 21 and the connection pipe 25 a release button 27 for separating the tongue 23 from the buckle 15 is provided on the buckle housing 14 the bag 12 of the shoulder belt 6 does not have a vent hole through which to discharge an extra high pressure gas but instead a relief valve 28 is integrally provided in an intermediate portion of the gas passage 20 of the buckle housing 14 the relief valve 28 includes a gas discharge hole 29 which allows the gas passage 20 to communicate with atmospheric air a valve disc 31 which is pivotally supported by a pin 30 and which opens or closes the gas discharge hole 29 and a spring 32 which urges the valve disc 31 in the valve closing direction the operation of the first embodiment of the present invention having the above described construction will be described below as shown in fig4 if the seat occupant connects the tongue 23 of the tongue device 8 to the buckle 15 of the buckle assembly 9 in order to strap on the air belt assembly the connection pipe 25 of the tongue device 8 is inserted into the connection port 19 of the buckle assembly 9 if the acceleration sensor detects an acceleration of not less than the predetermined value at the time of a collision of the vehicle the inflator 11 ignites to produce a high pressure gas and the produced high pressure gas is supplied to the bag 12 of the shoulder belt 6 through the gas passage 20 of the buckle housing 14 and the gas passage 26 of the tongue.