A pair of distance measurement sensors provided at different heights measures distances to an obstruction. When the measured distances are shorter than a threshold distance, the height of the vehicle is found based on the measured distances. When the height of the vehicle is found greater than a threshold height, the existence of the obstruction is reported to the driver. When the height of the vehicle is found smaller than the threshold height, on the other hand, a change in obstruction height is found and compared with a threshold. When the change in obstruction height is greater than the threshold, the existence of the obstruction is reported to the driver since the height is determined to be an unreliable result of computation.