1. A method of identifying a contour line comprising the steps of: searching a center position candidate point of a substantially circular contour line among input picture images in a predetermined field of view in which said substantially circular contour line is present; defining a region bounded by a circle having a center at said center position candidate point and a minimum permissible radius which is determined based on said searched center position candidate point and the radial distance from said point to the contour line, and a circle having a maximum permissible radius larger than said radial distance; scanning picture images in said region in a plurality of radial directions extending from said center position candidate point; detecting presence and absence of contour candidate points at which brightness changes abruptly during the scanning; and identifying that said substantially circular contour line is present in the picture images of said region when the number of said contour candidate points exceeds a predetermined set number.