Performing NURBS (non-uniform rational B-spline) interpolation machining is performed by reading cutter location (CL) data consisting of a tool control point vector and a tool axis vector on a workpiece coordinate system, converting the CL data into a position vector of three linear axes and a rotation angle on a machine coordinate system in accordance with the machine configuration of a simultaneous multiple-axis NC machine, calculating knot vectors of a NURBS curve with the most suitable chord length on the basis of a position vector of the three linear axes and a rotation angle, calculating a NURBS curve of the three linear axes and one rotary axis using the knot vectors, converting the NURBS curve into NURBS interpolation NC data, converting the feed rate on a workpiece coordinate system into the feed rate per minute on a machine coordinate system, and transmitting NC data to a NC machine.