An airbag having a cushion part defining an insertion hole that is positioned between first and second sewing lines provided to a base fabric and that is provided for the insertion of the inflator from an outside to an inside. A diffuser positioned within the cushion part and in which the inflator is inserted from the insertion hole. The diffuser has a tubular portion with a portion formed by folding layers of fabrics and being sewn to the base fabric along the first sewing line. The diffuser fabric of the tubular portion on a side adjacent to the base fabric being additionally sewn to the base fabric along a second sewing line. The diffuser includes a guide member 1 that extends and covers the insertion hole with a surface that continues from an inner surface of the diffuser. The guide member being sewn to the base fabric along the first sewing line.