Right and left side pressure sensors are provided to left and right doors, respectively, of a vehicle to sense a pressure in a door interior space of the corresponding door. An ECU computes a corrective pressure change amount that is obtained by correcting a change amount of a pressure, which is measured with a collided side one of the pressure sensors arranged in a collided one of the left and right doors that is collided with an object, based on a pressure, which is measured with a non-collided side one of the pressure sensors arranged in a non-collided one of the left and right doors that is not collided with the object. The ECU determines occurrence of the collision of the object based on the corrective pressure change amount.