1. A method for detecting from a digital chest image a ribcage boundary for computer-aided diagnosis with which anatomical structure information is detected from the digital chest image, including repetitions each comprising the steps of: deciding a size of a searching ROI mapped on the digital chest image, the searching ROI being deferent in position repetition by repetition; setting the searching ROI on the digital chest image; enhancing an image within the searching ROI set on the digital chest image; obtaining a profile of pixel values existing within the searching ROI; and analyzing the obtained profile so as to decide a candidate point for the ribcage boundary, wherein the deciding, setting, enhancing, obtaining, and analyzing Steps are repeated until a center of each searching ROI advances immediately before a given search limit, thereby a series of candidate points for the ribcage boundary being provided.