An airbag device comprising: an airbag attachable to an upper part of a side wall inside a vehicle cabin in the front-rear direction of a vehicle; and an inflator configured to supply inflation gas to the airbag for inflating and deploying the folded airbag toward the lower side of the vehicle cabin, wherein the airbag includes a two-folded portion and an accordion-folded portion continuous with the two-folded portion, wherein the two-folded portion is formed by folding a roll-folded portion into a U shape, characterized in that the roll-folded portion is formed by folding the airbag in a first direction with a roll width from a lower edge of the airbag toward an upper edge of the airbag, making a predetermined number of folds, the lower edge is opposite from the upper edge, which is to be attached to the upper part of the vehicle, wherein the roll-folded portion is folded in another second direction at a midpoint (c) of the roll-folded portion in the roll width direction so as to form the two-folded portion, and the accordion-folded portion is folded in an accordion manner, the accordion-folded portion is folded so as to be located at a side of the two-folded portion, wherein the accordion-folded portion is formed by bending a portion extending from an intermediate position (c) of the two-folded portion toward the lower edge of the airbag along the two-folded portion and then folding back toward the upper edge of the airbag at a folding line having the same width as the width of the two-folded portion, and the accordion-folded portion is located at a side to be faced to the vehicle-exterior of the two-folded portion, wherein an initially inflated part of the two-folded portion is folded so as to be stored between the two-folded portion. an airbag attachable to an upper part of a side wall inside a vehicle cabin in the front-rear direction of a vehicle; and an inflator configured to supply inflation gas to the airbag for inflating and deploying the folded airbag toward the lower side of the vehicle cabin, wherein the airbag includes a two-folded portion and an accordion-folded portion continuous with the two-folded portion, wherein the two-folded portion is formed by folding a roll-folded portion into a U shape, characterized in that the roll-folded portion is formed by folding the airbag in a first direction with a roll width from a lower edge of the airbag toward an upper edge of the airbag, making a predetermined number of folds, the lower edge is opposite from the upper edge, which is to be attached to the upper part of the vehicle, wherein the roll-folded portion is folded in another second direction at a midpoint (c) of the roll-folded portion in the roll width direction so as to form the two-folded portion, and the accordion-folded portion is folded in an accordion manner, the accordion-folded portion is folded so as to be located at a side of the two-folded portion, wherein the accordion-folded portion is formed by bending a portion extending from an intermediate position (c) of the two-folded portion toward the lower edge of the airbag along the two-folded portion and then folding back toward the upper edge of the airbag at a folding line having the same width as the width of the two-folded portion, and the accordion-folded portion is located at a side to be faced to the vehicle-exterior of the two-folded portion,