1. An image distortion correcting method comprising: analyzing color levels of all pixels of an input original image; determining a variation of the color levels depending on positions of the pixels; adjusting the variation of the color levels in a region where the variation departs from a predetermined threshold value; correcting the color levels of the pixels in accordance with the adjusted variation; and outputting the corrected image, wherein the adjusting of the variation of the color levels includes adjusting the variation of the color levels only when the variations of the color levels of red, green, and blue are equal or similar to each other.