To prepare an interpolation curve to be a smooth trajectory by matching a direction vector at the starting and ending points of the interpolation curve with a direction vector in the case of interpolation with a straight line in order to prevent a cut plane from having a dent and a flaw.

This interpolation method is characterized in that a first quadratic curve for deriving an interpolation curve for interpolating a gap between points P0 and P1 is replaced with a straight line through the points P0 and P1 when starting the interpolation curve, and a second quadratic curve for deriving an interpolation curve for interpolating a gap between points Pn-1 and Pn is replaced with a straight line through the points Pn-1 and Pn when ending the interpolating curve.