PURPOSE: To control calculation accuracy as needed by designating a distance from the position of a point, where a curve close to a contact state crosses a curved surface, to an approximate cross point as a converged result up to the moment.

CONSTITUTION: Under the condition of a contact P, I-th convergent point P1 on a curve A, (I+1)-th convergent point (improved point) Q2 on a curve B and cross point Q2 between the tangential line of the curve B and a straight line P1Q2 at the contact P, when the convergent point P1 is enough close to the contact P, a curved arc PP1 can be approximated to a circular arc and therefore, the condition of PQ2'=P1Q2' is established. Accordingly, since the condition of the straight line PP1<PQ2'+P1P2'=2*P1Q2' is established, by calculating a straight line P1Q2' the distance between the contact P and the straight line P1, namely, error can be evaluated. On the other hand, when there is the cross point, the condition of PQ2'<P1Q2' is established and therefore, the same formula is established as well. Thus, the cross point can be calculated with the accuracy as intended.