a side airbag unit for mounting on or in the backrest of a vehicle seat includes an airbag skin enclosing a gas chamber , and an inflator for filling of said gas chamber . the airbag skin exhibits a main section which serves for protection of the vehicle occupant when the side airbag is expanded and which in the resting state of the side airbag is present as a package , wherein the package is at least partly rolled . in order to provide a side airbag unit of low weight with its airbag skin nevertheless showing a controlled and reproducible expansion behavior , the airbag skin further comprises at least one auxiliary section which in the resting state encompasses the main section in two layers at least in sections , wherein an additional area of the gas chamber is present between the two layers of the auxiliary section at least in the resting state .