1. A point-sequence approximation method of approximating a point sequence constituted of a plurality of points by a line, comprising: inputting point data from a sensor; determining with a processor whether a proportion of a straight-line approximation region of the point data is high as compared to that of a curve approximation region; when 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 evaluating an 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 with the processor straight-line approximation using the candidate of the approximation straight line with the best result of evaluation of the approximation accuracy as an approximation straight line.