An airbag includes on an area of a circumferential wall encircling mounting holes a reinforced area that has a laminated structure of base cloths, i.e. a body cloth and a plurality of reinforcing cloths overlaid on the body cloth. Each of the base cloths has an adhesive layer on one surface and, the reinforced area includes in between the base cloths an adhesion area that bonds the base cloths together by heat sealing. The adhesion area is formed between two of the base cloths immediately adjoining each other or between two of the base cloths that has at least one other base cloth placed in between and at an area of the other base cloth where there is no cloth. All the base cloths are bonded together by the adhesion area without slippage from one another in a direction orthogonal to a layering direction of the base cloths.