There is provided a passive safety device which can prevent an occupant from moving when an acceleration that causes the occupant to move in a vehicle is working on the occupant, and which can improve the ride comfort. A passive safety device of the present invention comprises an acceleration detecting unit that detects an acceleration of a vehicle, a seat adjusting unit that expands a seat side of a seat in order to prevent an occupant sitting down the seat from moving, a safety belt adjusting unit that controls tension of a safety belt worn by the occupant, and a control unit that controls an expansion of the seat side and/or the tension of the safety belt through the seat adjusting unit and the safety belt adjusting unit, based on the detected acceleration of the vehicle.