PURPOSE: To prevent a tool holder from slipping out of a main shaft even in the case when cylinder pressure for clamping the tool holder to the main shaft decreases or comes to zero, as well as suppressing vibration of the main shaft in high speed rotation.

CONSTITUTION: A clamp rod 12 which is supported by being inserted in a main shaft 2 is moved forward and backward by means of a cylinder 16 and a piston 17 for clamping or unclamping a tool holder to or from the main shaft 2. The main shaft 2 is rotated by means of a driving motor 6 by keeping the tool holder clamped. Channels 24 and 25 extending radially are formed on the internal circumferential face of the cylinder 16 and on the outer circumferential face of the piston 17 respectively for housing movably locking bodies 26 therein. A static pressure coupling is prepared between the main shaft 2 and a motor shaft 10 for transmitting torque through a pressure fluid.