1. A method of extracting a specified image subject which successively implements a plurality of specified image subject extracting algorithms, comprising the steps of: implementing an extracting algorithm of a precedent stage under a predetermined extracting condition to obtain an extraction result; changing an extracting condition of a subsequent stage so as to be adapted to the thus obtained extraction result; and implementing an extracting algorithm of said subsequent stage under the thus changed extracting condition, wherein said precedent stage comprises extracting a shape of regular geometric form of the specified image subject.