PURPOSE: To shut off surely transmission torque from a drive unit to a bit with set torque and prevent power from superfluous consumption by providing the drive side can means parting axially from each other when clamp torque reaches a set value and a drive force from the drive unit acts.

CONSTITUTION: When clamp torque reaches a set value, a clutch ring 8 forcibly moves a clutch sleeve 20 through a stop shaft 5 similarly toward the bit end against a force of a torque spring 16 urging the clutch sleeve to the clutch shaft 3 side with the cam means 3a, 8a immediately after said cam means 3a, 8a part from each other so that a portion of the clutch sleeve 20 contacting a parting ball 11 provides a stepped small diameter portion 20". Thus, the parting ball 11 fitted in a cam ring groove 9' is removed therefrom and the cam ring 9 is urged by a spring 7 to be moved until it engages a stopper ring 10. The transmission of a drive force to the bit 19 is shut off by the parting of the cam means 3a, 8a from each other to stop clamping of a screw.