An airbag in which no choking occurs and which can be rapidly deployed. A cushion section of the airbag internally accommodates a cylinder-type inflator that includes an insertion hole for inserting the inflator from an outside to an inside, and a diffuser into which the inflator is inserted, and a diffuser having a tubular portion obtained by folding back a section of cloth to form symmetrical portions with an upper base formed by the folded back part and a lower base formed by adjacent ends of the symmetrical portions. The cushion section is folded back at both lateral sides of the along first folding lines that are substantially orthogonal to a longitudinal direction of the diffuser.