A folded airbag includes an outer circumferential wall which has a passenger side panel section disposed at a passenger side when inflation of the folded airbag is completed and a vehicle body side panel section extending forward from the passenger side panel section when the inflation of the folded airbag is completed, and an inlet opening which is positioned at a front lower side of the vehicle body side panel section when the inflation of the folded airbag is completed. The folded airbag is formed by folding an unfolded airbag body through front-to-back reduction folding that reduces a size of the unfolded folded body in a front-to-back direction so that a length of the folded airbag in a lateral direction is greater than a length of the folded airbag in a front-to-back direction.