1. A method of decoding a mixed code made by overlapping a first code image with a second code image, comprising: (a) receiving the original image which includes the mixed code; (b) obtaining a mixed code image by removing noise from the original image; (c) dividing colors, shades, and brightnesses of pixels of the mixed code image into groups using a predetermined threshold, and dividing the mixed code image into the first and second code images based on the groups, wherein the first code image includes construction information regarding the second image; and (d) decoding the first and second code images to extract first and second information, respectively, wherein (b) comprises: (b1) correcting distortion of the color and shade of the original image, which is caused by physical ambient conditions; (b2) producing a binary image by dividing one of the corrected color and shade into two colors based on a predetermined reference value; and (b3) obtaining the mixed code image by removing noise from the original image based on the binary image, and wherein (b3) comprises: (b3-1) determining a region contacting an edge of the binary image as noise and removing the region; (b3-2) dividing the binary image from which noise is removed into predetermined block units and detecting a block with a greatest number of pixels from the divided blocks; (b3-3) measuring maximum and minimum location values of the binary image in upper, lower, left, and right directions from a center of the detected block to an outside of the detected block or from the outside to the center; (b3-4) deriving a limited rectangle having vertexes of the measured maximum and minimum location values; (b3-5) extracting a mixed code image region in the limited rectangle; and (b3-6) deriving the mixed code image from the original image based on the extracted mixed code image region.