PURPOSE: To perform high precision machining with less error by previously measuring a difference between an actual tool holder position and an ideal tool holder position when the same tool is mounted at another turret position to connect a turret position correction storage to a correction data extractor.

CONSTITUTION: A turret position detector 1 detects a turret position, and position data is sent from the turret position detector 1 to a correction data extractor 2. The correction data extractor 2 extracts necessary correction data for a turret position error correction storage 3. The turret position error correction storage 3 stores a predetermined difference between an actual tool holder position and an ideal tool holder position when the same tool is mounted at another turret position. Correction data in the turret position error correction storage 3 is input from the correction data extractor 2 to an external machine coordinate shifter 4 for an NC lathe so that machine coordinates are shifted by a turret error correction to perform position correction of a tool.