If the maximum time width Lt of the intensity of received light of reflected
waves
from a vehicle ahead is smaller than a reference time width, it is judged that
the vehicle ahead is positioned in proximity to the detection limit distance of
an obstacle detection device for vehicle. Thus, there is no problem, for example,
even if a cut-in vehicle is present in reality between the vehicle ahead and the
vehicle of interest and nevertheless, the distance to the cut-in vehicle cannot
be detected. It can be judged whether the vehicle ahead is positioned in proximity
to the detection limit distance of the device by judging the magnitude relation
between Lt and the reference time width. As a result, the detecting capability
of the device can be judged with accuracy.
