of the retainer 52 and the flange 64 a distal end side of the inflator 58 passes through a central opening 52 b of the main plate portion 52 a of the retainer 52 and projects forwardly from the retainer main plate portion 52 a the distal end side of the inflator 58 is provided with gas spouting holes 58 a a rear end of the airbag 54 is fixed to the frame 60 by holding members 70 such as nuts one end of the tether belt 72 is connected to the distal end side of the airbag 54 and the other end of the tether belt 72 is connected to the frame 60 when the vehicle collides the inflator 58 is actuated and the module cover 56 is opened and torn so that the airbag 54 is inflated and the tether belt 72 is stretched tightly at this time in case the vehicle collision speed is more than a predetermined value or in case a weight of the occupant in the driver 39 s seat is more than a predetermined value voltage is applied to the piezoelectric displacement elements 66 so as to shorten the length of the rod 62 of the piezoelectric displacement elements 66 in the longitudinal direction accordingly the frame 60 is moved in a direction of an arrow s so that the protruding length of the airbag 54 from the retainer 52 is adjusted incidentally as the collision speed becomes faster or as the weight of the occupant becomes heavier the voltage applied to the piezoelectric displacement element 66 may be increased to increase an amount of movement of the frame 60 also the piezoelectric displacement.