An air bag comprises inside and outside base fabrics that are stitched to constitute an outer bag that is flat when receiving no gas. The outer bag is formed with a gas inlet opening through which an inflation gas is led into the outer bag to expand the same. The air bag further comprises inside and outside smaller fabrics that are stitched to constitute an inner bag that is installed in the outer bag. Given portions of the inside and outside smaller fabrics are stitched to given portions of the inside and outside base fabrics respectively to form a plurality of narrow auxiliary chambers between the inner bag and the outer bag.