PROBLEM TO BE SOLVED: To surely and completely inflate a gas bag before a side structural part is brought into contact with even a part of an occupant in a vehicle without impeding the complete development of the gas bag. SOLUTION: An impact protective device which is mounted on a back 1 of a seat 2 of an automobile is provided with an airbag module having an airbag and an inflator, an ignition device, and a cap 3 which covers a fitting opening part, is hinge-operable from the closing position to the opening position, and permits a gas bag to be developed forward without being impeded in the opening position. The cap 3 is mechanically reinforced, the cap is hinge-operable from the closing position to be directed approximately parallel to the axis in the longitudinal direction of the automobile to the opening position to be directed approximately perpendicular to the axis in the longitudinal direction of the automobile, and a stable separating means is formed between the back 1 of the seat 2 of the automobile in the opening position and a vehicle body part 6 opposite to the back.