A bag 1 is formed of an occupant-side sheet 2 and a vehicle body-side sheet 3. These sheets 2, 3 are superposed on each other and joined together along their peripheries so as to form an introduction chamber 7 and a main chamber 8 between the sheets 2 and 3. A lateral connecting line 10 is connected to the front side of the main chamber 8 and extends rearwardly from the end of the aforementioned edge in the substantially horizontal direction. The rear end of the connecting line 10 is spaced apart from the rear edge of the main chamber 8. A vertical connecting line 22 extends downwardly from the rear end of the lateral connecting line 10 and the vertical connecting line 21 extend downwardly from a portion of the lateral connecting line 10. The lower ends of the vertical connecting lines 21, 22 are spaced apart from the lower edge of the main chamber 8. Cells 8b, 8c, 8d communicate with each other through portions beneath the vertical connecting lines 21, 22.