1. A method for detecting a ribcage boundary for computer-aided diagnosis requiring anatomical structure information to be detected from a digital chest image, comprising the steps of: obtaining a profile of smoothed pixel-value integrated averages in each of right and left lung field of the image; deciding a threshold for each of the right and left lung fields with the profile taken as a reference; and searching each of the right and left lung fields from a central part of each lung field outwardly on the chest image so as to determine a position exceeding the threshold, the position being set as an outer search limit point for a series of upper lung ribcage boundary candidate points for the ribcage boundary.