During a period until a subject vehicle stops to go back for being parked, detection distance data items and advance distance data items are obtained by using an ultrasonic sensor and a speed sensor. These data items are stored with both the data items mutually associated. The detection distance data item indicates a distance to a parked vehicle adjacent to the subject vehicle and the advance distance data item indicates a distance in which the subject vehicle advances. When computing a size of the parked vehicle from the advance distance data items, the size of the parked vehicle is determined to be shorter than an aggregated distance corresponding to the advance distance data items. This enables the determined size of the parked vehicle to approximate an actual size of the parked vehicle.