PURPOSE: To improve the working properties and to prevent any artificial miss from occurring by calculating the center position of a workpiece on the basis of a coordinate value to a machine tool zero position at each measured position measured respective two points in each of X and Y directions, and making it so as to be registered in a control unit, in the case of a metalcutting machine tool.

CONSTITUTION: A workpiece 12 is clamped to a table 2, a measuring instrument 13 is attached to a spindle 8, and each registering position in both X and Y directions and another registering position of a center position of the workpiece 12 are specified to a numerical control system 9 from an operation panel 11. The table 2, a column 3 and a spindle head 6 are operated, making the measuring instrument 13 come into contact with one end in the X direction of the workpiece 12. Then, coordinates of the table 2 at that time are read out and registered. Likewise, coordinates at the other end in the X direction and at both ends in the Y direction are measured and registered each. In succession, with the operation panel 11 operated, the center position of the workpiece 12 is calculated on the basis of these registered coordinates, thus it is registered to a specified position in the control unit 9.