In a monitoring apparatus, a disparity calculator obtains, for each point of a target object, a pair of matched pixel regions in respective base image and reference image corresponding to the point of the target object. The disparity calculator calculates a disparity of each of the matched pixel regions of the base image relative to the corresponding matched pixel region of the reference image. A distance calculator corrects the calculated disparity of each of the matched pixel regions of the base image in accordance with a tolerable disparity range for the disparity to thereby increase the calculated disparity of the corresponding one of the matched pixel regions of the base image. The distance calculator calculates the depth distance of each point of the target object relative to the vehicle as a function of the corrected disparity of the corresponding one of the matched pixel regions.