PURPOSE: To provide an NC data altering system through which data having relevancy to data to be altered can be automatically altered.

CONSTITUTION: In an NC data altering system dependent on an automatic programming device, relevancy related to the alteration of data is previously set up and stored (SI), and the relevancy related to the alteration of stored data is used to read out the data related to the altered data in a NC device (S), and the read out data are automatically altered in correspondence to the altered data according to the relevancy (S4), and the NC data are altered on the altered data and the automatically altered data (S5, 6).