PURPOSE: To perform copy machining in a highly accurate and efficient manner, by outputting the command position data of a machine body, which is given by calculating a coordinate value of a following point behind in the central point of a stylus sphere at every sompling time, to a driving control part of the machine body after delay for the specified time.

CONSTITUTION: Data for positional coordinates X, Y and Z in a tracer and tilt angles α and β as well as a displacement value ez are outputted to an operation part 13 from a control part 12 for tracer driving, while those of command position data Xa, Yc and Zc of a machine body are outputted to a control part 11 from the operation part 13. When coordinates in the central point of a stylus sphere seat are set down to X, Y and Z, coordinates Xp, Yp and Zp in the central point of a stylus sphere are calculated whereby data for a coordinate value of a following point behind in the central point of the stylus sphere as slow as the specified valve are calculated as the command position data of the machine body so that such machining as being small in the amount of excessive cut due to overshooting is made performable.