An occupant detection apparatus can include a photographing device disposed to face to a vehicle rear seat for taking three-dimensional images; an information extracting section which extracts image information corresponding to an upper area of a seat of the rear seat from three-dimensional images taken by the photographing device; a division processing section which divides information extracted by the information extracting section into areas relating to respective sitting areas for occupants sitting in the rear seat; and a derivation processing section which derives, with regard to histogram showing frequency distribution of picture elements relative to distances from the photographing device, a first histogram corresponding to image information divided by the division processing section and also derives a difference histogram by deriving a second histogram from the derived first histogram, and thus derives information about each rear-seat occupant in each sitting area of said vehicle rear seat based on the difference histogram.