A partial image section is set as including part of a contour line of a particular image area in an original image appearing on a display screen. An image in the partial image section is binarized by referring to a selected threshold level. A boundary of the resulting binary image is detected for extraction of contour data in the partial image section. A plurality of such partial image sections are set successively in a way to partly overlap each other. Contour data (N2,M2) in an overlapping region of two adjacent partial image sections (Ni,Ni-1) are extracted by obtaining a plurality of dividing points (m.sub.0,n.sub.0,m.sub.1,n.sub.1, etc.) dividing, in corresponding proportions, two partial extracted contour lines lying in the overlapping region of adjacent partial image sections, distributing segments linking corresponding dividing points on the two parallel extracted contour lines in ratios corresponding to an order of arrangement of the dividing points, and deriving contour data for the overlapping region from coordinates of these distributing points (i.sub.1, i.sub.2, etc).