To sense impact in a side direction of a vehicle without adding a special specific sensor for detecting side impact.

The side impact sensing is carried out by using a speed sensor, a steering angle sensor of a steering wheel, and a yaw rate sensor for detecting rotations about a vertical axis of a vehicle. First, the speed of the vehicle 10 is detected (S10), and whether the vehicle 10 is stopped or not is determined (S12). While the vehicle travels, the yaw rate sensor 30 determines impact (S14), and it is determined whether ΔY/Δt which is hourly variation of a yaw rate Y is not less than a threshold value or not (S16). If the vehicle is determined to stop, the steering angle sensor 28 and the yaw rate sensor 30 carries out impact determination (S18), and it is determined whether a steering angle/Δt and the ΔY/Δt are not less than the threshold values (S20). When exceeding threshold values in each case, system main relay is intercepted (S22).