1. A method of detecting borders in a chest image, comprising steps of: calculating the gray level difference between neighboring pixels of the chest image to get a differential image; filtering the differential image to reduce noise and enhance target by at least one filter; binarizing the filtered image; picking out outside edges of an area with high gray level from the binarized image; and determining a final border from the edges according to the character of a border to be found, wherein said step of determining a final border from the edges is to determine a right heart border and a left heart border by detecting a right ribcage border and a left ribcage border, detecting a right diaphragm border and a left diaphragm border on the basis of the detected right ribcage border and the detected left ribcage border, and detecting the right heart border and the left heart border on the basis of the detected right diaphragm border and the detected left diaphragm border, to detect a right heart border so that a height difference between a right heart borderline candidate center and a right diaphragm borderline center is not less than 1/32 of the chest image height, and the horizontal difference between the heart borderline candidate center and the point that is the nearest point of the diaphragm borderline from a midline is less than ⅙ of the chest image width.