A distance measuring sensor is arranged to a vehicle door, and measures a distance to an object in a direction corresponding to a scanning angle. A memory stores a standard angle and a comparison distance corresponding to the scanning angle. A distance-shortest scanning angle is defined by the shortest value of the measured distance. When the distance-shortest scanning angle is different from the standard angle, the scanning angle is corrected. The object is determined to be an obstacle having a possibility to contact with the door, when the measured distance is equal to or shorter than a corrected comparison distance corresponding to the corrected scanning angle.