PURPOSE: To prevent a work main shaft from dislocation by furnishing a clamping means with a clamp adjusting means, which varies the clamping force of the clamping means in steps.

CONSTITUTION: A work main shaft 19 is equipped with a clamping means 11 having a plurality of braking means, which can clamp the rotation of the work main shaft 19, and contacting is accomplished with No.1 clamping force until all braking means come in contact with the work main shaft 19 in case it is clamped under C-axis control. After all braking means come in contact with the work main shaft 19, contacting is accomplished with No.2 clamping force (normal) which is stronger than No.1 clamping force. This prevents the work main shaft 19 from dislocation, which might be generated by single sided contacting of the braking means in the initial period of clamping in which the clamping force of the braking means is out of balance.