An object detector which is capable of discriminating a passage and an obstacle from each other easily even when such a discrimination with reference to a distance image alone is difficult due to the inclination of a traveling movable body and the passage in a slope. On the basis of the two-dimensional coordinate position data on each pixel of a distance image and distance data on a reference position of each pixel, three-dimensional coordinate position data in a three-dimensional coordinate system are computed with respect to each element of the distance image, whereby the three-dimensional distribution of pixels corresponding to a plane and an object to be detected is determined. Therefore, an object to be detected corresponding to pixels of not less than a predetermined height on the basis of the height of the plane can be discriminated easily from other pixels among the image elements corresponding to the plane, on the basis of this three-dimensional distribution of the pixels.
