An apparatus for acquiring a differential image includes: an image acquiring unit acquiring a first image including environment light from an image sensor of a camera and infrared polarized light from an illuminating unit and a second image including horizontal polarized light of the environment light; an image correcting unit correcting brightnesses of background areas of the first and second images; and a differential image generating unit generating a differential image between the first and second images of which the brightnesses of the background areas are corrected.