A side airbag device comprising: a seat frame that is along a side surface of a seatback of a vehicle seat, the seat frame being built in the seatback; an inflator that is attached to an occupant side of the seat frame; and an airbag cushion that receives a gas from the inflator so as to be expanded and deployed, wherein the airbag cushion includes: an inner side expansion part that is stored at the occupant side of the seat frame by being wound, the inner side expansion part being connected to the inflator; and an outer side expansion part that is continuously provided with the inner side expansion part, the outer side expansion part is stored by being wound at an opposite side of the seat frame with respect to the occupant side, the outer side expansion part receiving the gas from the inner side expansion part so as to be expanded and deployed; wherein the airbag cushion has a first folded-back part that is located between the inner side expansion part and the outer side expansion part, and the airbag cushion is folded back along a front edge of the seat frame with respect to the first folded-back part characterized in that the inner side expansion part includes: an inner contact part that directly comes in contact with the seat frame or indirectly comes in contact with the seat frame by sandwiching a cover between the inner contact part and the seat frame and extends from a position to which the inflator is attached toward a front edge of the seat frame; a third folded-back part with which the inner side expansion part is folded back from a front edge at a vehicle front side of the inner contact part toward a vehicle rear; and an inner folding part that further extends from the third folded-back part toward the front edge. a seat frame that is along a side surface of a seatback of a vehicle seat, the seat frame being built in the seatback; an inflator that is attached to an occupant side of the seat frame; and an airbag cushion that receives a gas from the inflator so as to be expanded and deployed, wherein the airbag cushion includes: an inner side expansion part that is stored at the occupant side of the seat frame by being wound, the inner side expansion part being connected to the inflator; and an outer side expansion part that is continuously provided with the inner side expansion part, the outer side expansion part is stored by being wound at an opposite side of the seat frame with respect to the occupant side, the outer side expansion part receiving the gas from the inner side expansion part so as to be expanded and deployed; an inner side expansion part that is stored at the occupant side of the seat frame by being wound, the inner side expansion part being connected to the inflator; and an outer side expansion part that is continuously provided with the inner side expansion part, the outer side expansion part is stored by being wound at an opposite side of the seat frame with respect to the occupant side, the outer side expansion part receiving the gas from the inner side expansion part so as to be expanded and deployed; the airbag cushion has a first folded-back part that is located between the inner side expansion part and the outer side expansion part, and the airbag cushion is folded back along a front edge of the seat frame with respect to the first folded-back part characterized in that the inner side expansion part includes: an inner contact part that directly comes in contact with the seat frame or indirectly comes in contact with the seat frame by sandwiching a cover between the inner contact part and the seat frame and extends from a position to which the inflator is attached toward a front edge of the seat frame; a third folded-back part with which the inner side expansion part is folded back from a front edge at a vehicle front side of the inner contact part toward a vehicle rear; and an inner folding part that further extends from the third folded-back part toward the front edge. an inner contact part that directly comes in contact with the seat frame or indirectly comes in contact with the seat frame by sandwiching a cover between the inner contact part and the seat frame and extends from a position to which the inflator is attached toward a front edge of the seat frame; a third folded-back part with which the inner side expansion part is folded back from a front edge at a vehicle front side of the inner contact part toward a vehicle rear; and an inner folding part that further extends from the third folded-back part toward the front edge.