1. A histogram stretching apparatus, comprising: an image analyzing unit receiving an input image to calculate a minimum image level and a maximum image level; a histogram generating unit generating a histogram of frequencies of image levels in a first section from the minimum image level to a first image level and a second section from a second image level to the maximum image level; an extension level setting unit comparing the frequencies of the image levels in the first section and the second section with a preset reference frequency to set an extension level; and a histogram stretching unit correcting the image levels of the input image according to the extension level, wherein the histogram generating unit adds a preset reference image level to the minimum image level to calculate the first image level and subtracts the preset reference image level from the maximum image level to calculate the second image level.