Satellite sensors are disposed in both sidewalls of a vehicle; each of the satellite sensors detects as an acceleration a collision of an obstruction with the corresponding sidewall. A floor G sensor is disposed in a central portion of the vehicle for detecting as an acceleration a collision of an obstruction with either of both the sidewalls. A two-dimensional correlational map is produced to have two parameters of an acceleration detected by the satellite sensor and speed variation derived from an acceleration detected by the floor G sensor. A collision determining threshold line is set on the correlational map. When a tracking line crosses the threshold line from the side containing the original point to the other side, a collision occurrence of a given pattern requiring activation of an occupant protection device is determined.