To prevent a spline curve passing through the point sequence from generating a large error with an original curve at the time of finely segmenting the curve and generating the point sequence by evaluating the error of a cubic spline curve and reconstituting the point sequence by adding a passing point inside a section in the case that the error exceeds an allowable value.

The curve is approximated fine segment data by the allowable error (S101). Then, the spline curve is generated from the (n) pieces of the passing points P1, P2,...Pn-1 (S102). Then, a specified point Pj is initialized (S103) and whether or not the inflection point of the curve is provided inside the section is evaluated in the respective sections of the spline (104). Then, in the case of judging that the inflection point is provided inside the section (S105), the error in the respective sections Pj-Pj+1 of the cubic spline curve is evaluated (S106). Further, whether or not the obtained error dj exceeds the allowable error is evaluated (S107). It is defined that the error is within the allowable value error and (S109) is advanced at the time of dj≤ and it is defined that it exceeds the allowable error and (S108) is advanced at the time of dj>.