A side-impact airbag device for a passenger cell of a motor vehicle includes a window bag which, in the active, inflated state, has a window portion which at least partially covers a side window and extends between roof support pillars. At least one of the window portions of the window bag is assigned an additional window bag pillar portion which, in the active, inflated state, at least partially covers the associated pillar relative to the passenger cell and provides a cushion against the impact, in particular, of the head and shoulder area of a passenger.