1. A point-sequence approximation method of approximating a point sequence constituted of a plurality of points by a line, comprising: determining whether a proportion of a straight-line approximation region is high as compared to that of a curve approximation region; if it is determined that the proportion of the straight-line approximation region is high as compared to that of the curve approximation region, repeatedly performing random setting of a candidate of an approximation straight line, and evaluation of approximation accuracy of the candidate of the approximation straight line based on distances between the candidate of the approximation straight line and the points constituting the point sequence; and performing straight-line approximation by using the candidate of the approximation straight line with the best result of evaluation of the approximation accuracy as an approximation straight line.