An airbag includes a body cloth made of polyamide woven fabric and constituting a circumferential wall of the airbag. The body cloth includes on the inner surface a bond area having a coating layer of polyamide elastomer and to which a supplementary cloth made of polyamide woven fabric is bonded, an extended area having a same coating layer of polyamide elastomer as on the bond area, and an uncoated area that has no coating layer. The coating layer on the bond area is used as heat sealable adhesive that adheres the supplementary cloth to the body cloth whereas that on the extend area is arranged in such a manner as to be exposed to inflation gas upon airbag inflation so as to act as a protective membrane for preventing gas leakage.