PURPOSE: To measure edge point position displacement of a tool under a condition that the tool mounted on a spindle is rotated at a rotational speed when actually machined.

CONSTITUTION: In an NC machine tool, a tool 3 during rotating by a rotational speed at actual machining time is moved to approach a measuring electrode 21a of an electrostatic capacitance displacement measuring device 21 by a moving command commanded from a numerical control part 7, and a position data of a feed shaft, when a clearance between the tool 3 and the measuring electrode 21a comes to be a prescribed reference value, is detected by a position detecting part 31 and fed out to an arithmetic part 23. The arithmetic part 23, fetching a tool data of the tool prestored to a tool data memory part 17 through a tool data feed part 15, calculates edge point position displacement of the tool 3 during rotating from the position data and the tool data.