1. An image processing method implemented by a computer programmed as an image processing apparatus for recognizing one or more quadrangles from an input image, the method comprising the steps of: (a) detecting, by the image processing apparatus, a plurality of edge areas from the input image; (b) extracting a plurality of lines, the lines corresponding to the edge areas detected; (b′) selecting a plurality of line pairs from the lines extracted at the step (b), generating a new line according to a positional relationship between two lines included in a line pair among the selected line pairs, and adding the new line to the lines extracted at the step (b); (c) categorizing into a category a line pair selected from the extracted lines according to a positional relationship between two lines included in the line pair, and calculating a line pair evaluation value for the line pair, wherein the step (c) includes selecting all line pairs from the lines extracted in step (b) and the new line generated in step (b′); (d) selecting a combination of two line pairs from a plurality of line pairs, generating a quadrangle from four lines included in the two line pairs selected, and calculating a quadrangle evaluation value for the quadrangle based on the categories and the line pair evaluation values of the two line pairs forming the quadrangle; and (e) selecting a quadrangle based on the calculated quadrangle evaluation value.