PURPOSE: To input exactly and speedily the shape of blank for a workpiece by installing a range finder with non-contact system which measures the distance to the surface of a workpiece through moving the workpiece along its axial direction.

CONSTITUTION: The feed rate of the table 52 of a grinding machine with a position control device is controlled, a workpiece is moved in its axial direction and the distance to the surface of the workpiece 58 is measured along its axial direction by a laser range finder 71 with non-contact system fixed to the bed 51 etc. of a grinding machine. Signals from the range finder 71 are input into the outside diameter operation part of a numerical controller 30 and the outside diameter of the workpiece 58 is operated. Based on the output signals from the outside diameter operation part, the outside diameter of the workpiece 58 along its axial direction is operated by the shape operation part of the numerical controller 30. The obtained outside diameter of the workpiece 58 is stored in the memory unit of the numerical controller 30 and the measured data of the outside diameter are used for displaying the shape of blank for the workpiece 58 on CRT 22.