PURPOSE: To prevent the delay of the judgement of an emergent condition in the case where the relatively small acceleration lasts for a long time by providing a judging means to judge that a vehicle is in an emergent condition when the integrated value exceeds a specific value.

CONSTITUTION: An accelerator sensor 14 is connected to a microcomputer 12 through an ADD converter 15 to convert the analog signal to the digital signal, and the acceleration of the vehicle is detected. The detected acceleration value of each waveform is integrated in terms of the time by a first integrating means of the microcomputer 12, and the square value of the detected acceleration is integrated in terms of the time by a second integrating means. An air bag actuator 16 is actuated based on these integrated values and the judgement by a judging means, and the air bag body 18 is inflated by the combustion gas generated by the explosion of the explosives or the gas which is preliminarily accumulated.