An airbag has a lower expansion portion, a central expansion portion, and an upper expansion portion. When the airbag is deployed, the lower expansion portion corresponds to the lumbar region of an occupant, the central expansion portion corresponds to the abdomen of the occupant, and the upper expansion portion corresponds to a shoulder of the occupant. The lower expansion portion is folded upward to be tucked in a pair of fabric sheets forming the central expansion portion. After tucking, the airbag is folded to form a bellows-like portion such that the distal end of the airbag approaches an inflator located in the proximal end of the airbag. After folding to form the bellows-like portion part of the bellows-like portion that corresponds to the upper expansion portion, which has not been tucked, is bent downward substantially by 180° so that the part contacts the front side of part of the bellows-like portion that corresponds to the central expansion portion. Accordingly, irrespective of the positions of a joined section or a narrow section, the packaged size of the folded airbag is made compact.