PURPOSE: To control the rotating speed of a main shaft to within the allowable maximum rotating speed and prevent the accident of a work getting out of place by making the main shaft speed control of a numerically controlled device automatically set the allowable maximum rotating speed in accordance with the size of a chuck.

CONSTITUTION: The main shaft speed contol of a numerically controlled device controls the speed of a main shaft motor 12 so that the rotating speed of a main shaft 14 will agree with the directed value. In this main shaft speed control, a memory is set to store the allowable maximum rotating speed corresponding to the discrimination result of a chuck discriminator 23 which functions to detect the size of a chuck 19 for fixing a work 18 on a head stock 17 of the main shaft 14. In addition, it is equipped with a device which clamps the speed value directed to the main shaft motor 12 at the allowable maximum rotating speed or issues an alarm when the direction is given over the allowable maximum rotating speed according to the discrimination result of the chuck discriminator 23.