1. A computer-readable recording medium that stores therein a computer program that causes a computer to execute extracting a set of parallel geodesic projections in which parallel geodesic projections, which are geodesics parallel to each other on a three-dimensional paper curved surface as a ruled surface, are projected on an image obtained by imaging the three-dimensional paper curved surface, from the image, and searching for and extracting a set of ruled line projections by using a condition that an interior division ratio determined based on a ratio of segment lengths obtained by dividing the rules line projections in which ruled lines, which are straight lines forming the three-dimensional paper curved surface, are projected on the image, respectively, by the set of parallel geodesic projections is constant, the computer program causing the computer to execute: first extracting including extracting 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 the sets of parallel geodesic projections, as a set of ruled-line candidate projections as a search target of a set of ruled line projections; calculating a deviation of neighborhood, which is a distance between a vector determined based on an interior division ratio of the ruled-line candidate projection extracted at the first extracting and a vector determined based on an interior division ratio of a neighboring line obtained by shifting the ruled-line candidate projection by a predetermined interval, for each of the ruled-line candidate projections; storing and holding the deviation of neighborhood calculated at the calculating for each ruled-line candidate projection; and second extracting including extracting a set of straight lines having the smallest sum total of the deviations of neighborhood held at the storing and holding as a set of ruled line projections, in the sets of straight lines that do not intersect with each other among the sets of ruled-line candidate projections, according to continuous dynamic programming.