1. An edge adjustment method for adjusting an edge of an input image using an image processing device, comprising the steps of: extracting edge components of the input image by the image processing device; calculating a sum of the extracted edge components by the image processing device; calculating an average of luminance components of the input image by the image processing device; determining, by the image processing device, an enhancement degree of the edge as a large value if the sum of the edge components is smaller than or equal to a threshold value and the average luminance is greater than or equal to a threshold value, and as a small value if the sum of the edge components is greater than the threshold value or the average luminance is smaller than the threshold value; adjusting the determined enhancement degree according to the extracted edge components by the image processing device; enhancing, according to the adjusted each enhancement degree, the each edge component corresponding to the each enhancement degree by the image processing device; and adjusting the edge of the input image according to the enhanced edge components by the image processing device.