Side airbag unit

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.