Both end portions in a second direction intersecting a first direction along which an airbag is deployed are folded to a center portion in the intersecting direction (first folding). The airbag is folded so that an upper side portion and a lower side portion are close to each other (second folding). The airbag has a bellows portion which is folded in a bellows shape between a connecting portion and a spiral portion. The airbag folded like this is enhanced in housing performance for the vehicle, and increases in stability of the deployment when the airbag is inflated and deployed.