Abstract: A rolling angular velocity sensor (30) is used to detect a roll of a vehicle. When the vehicle rolls at a predetermined or greater rolling rate and a rolling angle RA becomes equal to or greater than a predetermined upper limit value RA1, a control unit (50) gives an instruction to a high-speed control valve (46) of an attitude-sustaining air bag (22) via an input port (56) to open the high-speed control valve (46). This enables the high-pressure air to be fed into an air bag body (48) of the attitude-sustaining air bag (22), which is accordingly inflated and expanded to support the attitude of a passenger in a passenger's seat. When the acceleration in the lateral direction is not sufficiently large, when the passenger fastens a seat belt (24), or when the passenger is not seated, the attitude-sustaining air bag (22) is not activated.