A knee airbag (100) which is deployed between the vehicle occupants and a vehicle structure (2) and a gas generator (4) for filling the knee airbag (100) with inflating gas, wherein the knee airbag (100) has a base chamber (10) which is filled first with inflating gas, and at least one further upper chamber (20, 30) which receives the inflating gas conducted through the base chamber (10). A flow obstacle (12, 23) for the inflating gas is arranged between the base chamber (10) and the one or more upper chambers (20, 30). The one or more upper chambers (20, 30) is rolled up or collapsed and is secured in the rolled-up or collapsed state, the securing (21, 22) being designed in such a manner that it fails when a predetermined degree of filling of the base chamber (10) is reached.