1. An automatic range finding method capable of measuring a distance between a stereo camera and a reference plane, the automatic range finding method comprising: acquiring a disparity-map video by the stereo camera facing the reference plane; analyzing the disparity-map video to generate a depth histogram; selecting a pixel group having an amount greater than a threshold from the depth histogram; calculating the distance between the stereo camera and the reference plane by weight transformation of the pixel group; and applying a coarse-to-fine computation for the disparity-map video.