To solve a problem that a non-inverted part is left behind so that an airbag is inverted in an approximately uniform manner toward an outer peripheral edge of a whole airbag body from the center of an opening in inverting the face side and the back side of the airbag body not only in a case where the distance from the center of the opening to the outer peripheral edge of the airbag body is approximately uniform, but also in a case where the distance is not uniform.

In an inverting direction of an airbag body 10 provided with an opening 14, an airbag base cloth 12 on the side without any opening 14 is clamped at a plurality of parts from both face side and back side making use of the opening 14, and the face side and the back side of the airbag body 10 are inverted through relative movement so that the clamped parts pass the opening 14.