To reduce the apex unevenness of a quadratic function despite a large distance ratio set between the continuous sequences of points and to obtain a smooth interpolation track.

If the spaces (distances) are not fixed among passing points of the continuous (m-1)-th, m-th and (m+1)-th tools between (n+1) pieces of sequence of points P0 (X0, Y0, Z0), P1 (X1, Y1, Z1)..., Pn (Xn, Yn, Zn) which are set as the tool passing points, the distance between the (m-1)-th point Pm-1 and the m-th point Pm is calculated together with the distance between the point Pm and the (m+1)-th point Pm+1. Based on the ratio set between both calculated distances, the coefficients of 1st and 2nd curves of second order are converted to secure the smooth interpolation between those tool passing points.