1. A stereo image matching device comprising: a left camera and a right camera which take images of the same position at the same time to provide a stereo image; a stereo matching unit which performs a matching process based on a correlation for an image block on the basis of a disparity search range for each image block estimated and divided for the previous disparity image in two images provided from the left camera and the right camera, and outputs a disparity image for the current stereo image; and a disparity search range estimating unit which generates a disparity histogram configured with the number of pixels for each disparity value with respect to the disparity image output from the stereo matching unit, divides the disparity image into multi-level blocks on the basis of the number of valid disparity values obtained by masking valid disparity values having the number of pixels equal to or more than a threshold value in the disparity histogram, estimates a disparity search range to include a valid disparity value for each divided image block, and provides the disparity search range to the stereo matching unit.