A process for determining lateral overturning of a vehicle involves establishing a threshold value line S separating a lateral overturning region farther from an origin and a lateral non-overturning region closer to the origin on a two-dimensional map made using a rolling angle .theta. and a rolling angular speed .omega. of the vehicle. When a hysteresis line for actual rolling angles and actual rolling angular speeds of the vehicle traverses the threshold value line from the lateral non-overturning region to the lateral overturning region, it is determined that there is a possibility of lateral overturning of the vehicle, and correspondingly an occupant protecting system may be deployed. Also, to assure accuracy, a lateral acceleration sensor for detecting a lateral acceleration for establishing the threshold value line S is disposed on a center plane of a vehicle body.