A set of straight lines that associate a top parallel geodesic projection positioned at an upper end with a bottom parallel geodesic projection positioned at a lower end, among sets of parallel geodesic projections, is extracted as a set of ruled-line candidate projections as a search target of a set of ruled line projections. A deviation of neighborhood, which is a distance between a cross ratio vector of the ruled-line candidate projection and a cross ratio vector of a neighboring line obtained by shifting the ruled-line candidate projection by a predetermined interval, is calculated for each ruled-line candidate projection. A set of straight lines having the smallest sum total of deviations of neighborhood, in the set of straight lines, which do not intersect with each other, among the sets of ruled-line projection candidates is extracted as a set of ruled line projections by continuous dynamic programming.