An apparatus of detecting an object on a road surface includes a stereo set of video cameras mounted on a vehicle to produce right and left images, a storage to store the right and left images, a parameter computation unit to compute a parameter representing road planarity constraint based on the images of the storage, a corresponding point computation unit to compute correspondence between a first point on one of the right and left images and a second point on the other, which corresponds to the first point, based on the parameter, an image transformation unit to produce a transformed image from the one image using the correspondence, and a detector to detect an object having a dimension larger than a given value in a vertical direction with respect to the road surface, using the correspondence and the transformed image.