The invention relates to an inertization device for establishing and maintaining an inertization level in a protective room. The inertization device has a controllable inert gas system for providing inert gas, a first supply pipe system connected to the inert gas system and the protective room to supply the inert gas to the protective room, and a control unit to control the inert gas system such that a presettable inertization level is established and maintained inside the protective room. In order to raise the inertization level inside the protective room rapidly to an accessibility level without requiring major structural measures, a valve controlled by the control unit is connected to the inert gas system and the first supply pipe system to supply the exhaust air prepared by the inert gas system as fresh air to the protective room.