An object of the present invention is to provide an air bag having a mechanical characteristic durable against the gas of high output and high temperature generated from an inflator which has been compacted in recent years while reducing amount of the gas leaked from sewn part of the air bag. The air bag according to the present invention is an air bag for vehicles comprising a main body part consisting of a base fabric and a sewn part being formed by sewing, with a sewing thread, the above main body part into a bag, characterized in that, the difference between the elongation of the sewing thread upon applying the tension of 31.3/a (N/thread) to the sewing thread and the elongation of the base fabric upon applying the tension of 62.5/a to the base fabric (N/stitch) [wherein "a" (stitch(es)/cm) is a sewing pitch in the sewn part] is within a range of 0 to 5.0% in both directions of warp and weft, and the air permeability of the sewn part at 40 kPa during the initial stage of development of the air bag is less than 50 mm/cm/sec.
